In 1984, American travelers reported 25,000 passports lost or stolen, the greatest number of them in the heavily visited countries of Great Britain, France and Italy. And in the same year, some 150,000 claimants worldwide applied to American Express for replacement of missing traveler's checks.

Certainly most of these losses represented at least a temporary inconvenience, if not outright hardship, during a business or vacation trip -- which is precisely when you want everything to go right. The moral, of course, is take good care of your valuables.

But what should you do if you become part of 1985's loss statistics? Here's a guide to quickly replacing passport, traveler's checks and cash so you can get on with your trip.

* Passport: U.S. passports are a valuable commodity, says the State Department's Bureau of Consular Affairs, and should not be left behind in a hotel room while you are sightseeing. If stolen, they could be altered and used by illegal immigrants and others, including terrorists, wanting to conceal their identities.

If your passport is missing, you must first notify the local police and obtain a copy of the loss report you file with them. The next step is to take the report, along with some proof of your identity, to the nearest U.S. embassy or consular office. Proof can be a driver's license, a copy of your birth certificate or some other identification with your photo on it. A photocopy of your passport, made before leaving home, could be of help.

If you have lost all your documents, another U.S. citizen with a passport -- your spouse or a traveling companion, for example -- usually can vouch for you. This means, of course, that one person should not carry all the passports for your family or group.

You will also need two passport photos and $42, the same fee you pay for a new passport back home. It hastens the paperwork if you have jotted down separately a record of your passport number.

With proper identification, the replacement passport should be ready the day you apply. When identification is in doubt, the consular office may have to get verification of your passport from the United States, delaying replacement for 24 hours or more.

* Cash: Carry traveler's checks instead; unlike cash, they generally can be replaced rapidly without financial loss to you.

However, in an emergency you can usually arrange to obtain extra cash from home overnight with help from the State Department's Citizens Emergency Center. This is how it works:

Contact the nearest U.S. embassy or consular office, which can help you phone or wire home for the money. To get the funds to you quickly, relatives or friends should send them via Western Union or Federal Express to the Citizens Emergency Center, Department of State, Washington, D.C. 20520, (202) 632-5225. Once the center has the money, it will notify the embassy or consular office to release an equivalent amount to you. The cost of the service, which is for emergencies only, is $15.

Some charge-card and traveler's-checks firms will permit their customers to write a personal check for immediate cash and traveler's checks -- among them, American Express and Citicorp -- after verifying you were issued a credit card or purchased checks. (Some firms also permit you to use a blank check form, if your own checks have disappeared with everything else.) Traveler's checks: They are the safest way to carry your money. And the incentives to do so are increasing.

One of the newest services to holders of traveler's checks was begun this month by American Express. In 70 U.S. cities, the company promises that missing checks will be replaced by special courier delivery within three hours of the time you phone -- every day, 24 hours a day, and at no extra charge. As an American Express spokesman says, "You don't even have to leave the swimming pool."

The process is smoother if you have retained your check receipts, which contain the check identification numbers, but the receipts aren't essential. To get courier delivery, phone American Express at (800) 221-7282. The operator will verify your check transaction by computer and contact the courier in the city you are visiting.

When you report a check loss, American Express also will arrange to cancel any lost or stolen credit or charge cards (not just the American Express card); set up new airline, hotel and car-rental reservations, if you have had to change plans; grant you a free telegram or international cable home to let someone know about the changes; and issue you a temporary identification card, until you can replace your driver's license or passport.

Citicorp is offering another kind of service, begun in January, to check purchasers. It doesn't have anything to do with check losses, but it could come in handy in an emergency. International travelers can phone collect to a special Citicorp number, (813) 879-7701, to obtain the name of the nearest English-speaking doctor or lawyer.

At the beginning of this month, BankAmerica inaugurated what it calls its "SafeTravel Network" for check purchasers. For an additional fee of $5 (individuals) and $10 (family of four), the company promises a consultation with an English-speaking doctor; a $1,000 interest-free loan if needed for hospital admission; referral to a local English-speaking attorney; and up to $200 a day for three days (for room and meals) if a delayed plane or similar occurrence interrupts your itinerary.

At no extra charge, holders of Visa traveler's checks can arrange to have Visa cancel any missing credit cards. Check holders also are allowed to wire home at no cost to report any change in itinerary caused by an emergency.

Whatever the extra services, traveler's checks are issued with information detailing what to do in case of loss, and each company has its own procedures. In the United States, customarily there's a 24-hour toll-free "800" number where you can report the loss and be directed to the nearest replacement office. Some arrangement usually is made for limited emergency funds to be obtained at a hotel or rental-car desk on weekends and holidays.

The same is true for reporting losses abroad, except that you may be directed to phone collect back to the United States (Citicorp) or call collect to special numbers in Europe and Asia (BankAmerica, Visa). American Express and Thomas Cook have their own offices worldwide; other traveler's-check firms will make replacements through authorized agents, such as local banks.

In any case, obtaining new checks is quicker when you have safeguarded the purchase receipts.

* Airline tickets: Treat them "like cash," says a spokesman for United Airlines. If a ticket is stolen or lost, you probably will have to buy a new one before you can board your flight. You will get a refund only if the missing ticket does not turn up after a certain specified time, as long as 60 or 90 days.

Unrestricted tickets, which can be used anytime, are most vulnerable. Those requiring advance purchase and carrying a penalty when not used on a specific day are less open to illegal use. The same is true for tickets on many international flights, since passport identification is usually required to board.

Missing tickets should be reported to the airline immediately, says the United spokesman. Getting the loss entered in an airline's computer makes fraudulent use less likely.

* Credit cards: Along with passport and traveler's-check numbers, you should make a list of the numbers of any credit cards you take on a trip, especially abroad. Any loss should be reported as soon as possible to one of the card firm's agents or its emergency number.

American Express and Diners Club say they can usually replace a missing card within 24 hours. Holders of Visa's "premier" card also can get emergency replacement, but other Visa card holders must wait until they return home.

HIGHLANDS HOTLINE: As a pilot project for British tourism, a telephone hotline to the Scottish Highlands, called the HiLine, has been established this year.

If your travel plans will take you to Scotland, you can phone ahead to a Highlands computer network to make hotel and transportation reservations, get help with an intinerary or ask about train schedules and fares. The direct-dial number is 011-44-349-63434.

The line is open from 9 a.m. to 5 p.m., British time, which means 4 a.m. to noon, Eastern Standard Time (Central, 3 to 11 a.m.; Mountain, 2 to 10 a.m.; Pacific, 1 to 9 a.m.). If the hours seem awkward, that's when phone rates are cheapest.

Until 7 a.m. (whatever time zone), the charge is $1.17 for the first minute and 71 cents for each additional minute. From 7 a.m. to 1 p.m., the charge is $1.95 for the first minute and $1.18 for each additional minute. The line was set up by the Highlands & Islands Development Board.