Right alongside the passport, airline tickets and traveler's checks, Americans vacationing abroad might want to consider some key health planning.

As many as one in every five travelers develops a health problem overseas, experts estimate, and one in 15 needs some type of medical care while outside the United States.

They are often shocked to discover that most foreign hospitals do not accept standard American health insurance coverage. Moreover, these hospitals often don't allow patients to be discharged until the bill is paid in full.

"Employe health policies may reimburse you for illnesses or accidents overseas, but almost no one abroad will accept your health insurance card," says Dr. W. Scott Harkonen, an internist in San Francisco and author of the book "Traveling Well."

Some people find that, when they return home, their policies still don't pay for health care expenses incurred abroad, or they are reimbursed at a lower rate. Medicare pays nothing for expenses outside the United States; Blue Cross-Blue Shield often reimburses at a lower-than-normal rate. The 80 million Blue Cross-Blue Shield policy-holders "will have to pay their own bills and collect when they get home," says company spokeswoman Arlene Flom. Medical bills incurred abroad are usually reimbursed "at a lesser amount" than bills in the United States because "costs are less" overseas.

"It's possible that people might not be aware of this policy," says Flom. "But it is in their contract if they read it."

Members of health maintenance organizations may have slightly more protection -- provided an illness or accident abroad is deemed an emergency. "Some health problems are judgment calls," says Jim Doherty, executive director of the Group Health Association of America, an industry group of HMOs. "But there is no question that out-of-area coverage for emergencies is paid for by HMOs."

The bottom line, says Harkonen, is that "if you get sick or are injured, you could be facing a big financial problem." To help with this gap in medical insurance coverage, several companies now offer special insurance plans for travelers. (See box at left.) At least one plan -- Access America -- will cover preexisting medical problems, provided a doctor says in writing that the condition is not likely to flare up during the trip.

Then there's the expense of being transported back home in case of serious illness. "The average cost of a medical evacuation worldwide is $10,000," says Harkonen.

The best bet is to reduce the risk of getting sick abroad. To maximize your chance for healthy travel, experts suggest planning and assessing your health before crossing the border. Start "at least four weeks in advance, and preferably eight," says Dr. Lawrence D'Angelo, director of the International Health Service at Georgetown University.

"Take a personal health inventory before you leave," adds Harkonen. "The time to detect personal health problems is at home."

If there is a preexisting health condition -- such as high blood pressure, diabetes or pregnancy -- travelers should make sure they have their doctors' approval for taking a trip. D'Angelo suggests carrying a written medical summary from your physician "listing medications, medical history and any medical complications."

This summary should be carried with your passport at all times, D'Angelo says, and will not only help if there is a recurrence of the illness but also can make it easier to get prescription medicines through customs.

People taking prescription drugs need to carry at least a 48-hour supply on their person during travel, plus have a back-up supply for the trip and a written prescription with the generic rather than the trade name. These measures should help get the drug properly refilled abroad if necessary.

"And remember dental health," says D'Angelo. "If you think it's hard to get a doctor abroad, it's even harder to find a dentist."

The list of immunizations required for traveling varies greatly with itinerary. One of the most widely recommended is gamma globulin, which will offer fairly good protection against hepatitis A. This injection needs to be given very close to departure date and is good for about six months. Other immunizations that need to be up-to-date include:

* Tetanus and diphtheria, which are given in childhood initially and should be administered about every 10 years to adults.

* Polio, also given in childhood. Many developing countries are experiencing outbreaks of this crippling disease.

For those visiting areas with malaria, yellow fever, typhoid, cholera and other such diseases, information on required immunizations is available from the World Health Organization, part of the United Nations in New York and with offices at the Pan American Health Organization in Washington. International Association of Medical Assistance for Travelers also has information, as do embassies, health clinics specializing in travelers' ills, the federal government's Centers for Disease Control in Atlanta and local physicians.

As for traveling with very young children and infants, Dr. Martin S. Wolfe, director of the private Travelers Medical Service and a consultant to the State Department and the World Bank, says that "there's no hard and fast rule.

"But it's desirable," he says, "for children to be at least a couple of months old before they travel."

Despite their best efforts, though, some travelers may come home with far more than fond memories. Certain illnesses may not be apparent until months after being back.

D'Angelo recalls one patient, a woman who had traveled to Puerto Rico for a four-day business trip. She developed dengue fever, a viral infection transmitted by mosquitos, after she had returned home.

The woman's infection was misdiagnosed -- one doctor thought that she had leukemia -- before it was correctly identified and she received the proper treatment.