The health-insurance hunting season opens tomorrow for millions of federal workers and retirees. More than half the people in the Washington area who have health insurance are covered by one of the plans in the government program, which is the nation's biggest company-style group health package. Most workers and retirees can pick from among about 20 fee-for-service plans or health maintenance organizations.

If you are a single nonpostal federal worker or retiree and have "average" medical costs next year, you can pay as little as $500 or more than $4,500 in out-of-pocket expenses for identical treatment and hospitalization. The $4,000 difference depends on which plan you pick during the open season that runs through Dec. 10.

Starting tomorrow, we will have a series of columns about the health plan choices, with rankings from experts in the health insurance field. Stay tuned during the open season, because you can save -- or waste -- a lot of money on insurance.

There is no such thing as a "best" health plan. All of them are rated as good to excellent. What you need depends on your age, family situation, health and finances. The lowest premiums are offered by the health maintenance organizations. But some people prefer traditional fee-for-service plans that let them select their doctor, hospital and treatment program.

Retirees with Medicare don't need to buy the same policy as the head of a young family. All of the plans provide catastrophic coverage. It kicks in when your out-of-pocket costs hit certain levels. Some plans provide dental benefits. We will cover them all during the next couple of weeks.

Experts agree that you don't need to pay high premiums to get good coverage. Also this year, all plans will require subscribers to get clearance for non-emergency hospitalization. That is standard in the insurance business. If you don't do it, you could be penalized $500.

Most plans have lifetime dollar limits on what they will pay for some things. But that is no problem for federal workers and retirees. They can change plans every year and start all over again with their new plan's lifetime limit. Another feature of the federal plan is that no employee, family member or retiree can be refused coverage because of preexisting medical conditions.

If you don't change plans, you will be kept in your current plan for 1991. The exception is for the 50,000 people who are now in plans that are leaving the federal program. They include the health plans offered by the National Association of Government Employees, American Federation of Government Employees, National Federation of Federal Employees and National Association of Postal Supervisors.

Their subscribers must pick a new plan. If they don't, they will be placed in a plan by the government. The good news is that whatever plan they are assigned to probably will be better than their current plan and cost less.

Good hunting!