(MONEY Magazine) – Have you been tempted to join your company's health maintenance organization (HMO) for its low cost but feel uncomfortable about giving up visits to your family doctor or to the top specialist in town in an emergency? You might have more flexibility than you think. More than 600,000 employees and their dependents have enrolled in a new type of HMO called an open-ended plan. This hybrid, offered by Allied-Signal, Martin Marietta and the State of Arizona, among many others, combines the standard HMO flat-fee coverage (typical premium: $100 to $260 a year) with a traditional group health insurance option known as an indemnity plan. The open-ended HMO gives you the freedom to see non-HMO doctors for a manageable extra fee. In a typical open-ended plan, if you bypass the HMO network in favor of an ''outside'' doctor, the cost of your visit is treated like a conventional major-medical insurance claim, with annual deductibles and co-payments. The expense of such trips can run 20% higher than if you had signed up for a standard indemnity policy. A Cigna open-ended HMO plan, for example, requires a $300 annual deductible for outside office visits before reimbursing 70% of those costs, with a $3,500 annual limit on the amount you could owe. By contrast, a traditional group health indemnity plan reimburses 80% after only a $200 deductible and pays everything above $2,000. Some open-ended HMO plans < also demand additional co-payments from patients on outside billings that the insurer deems higher than ''customary.'' Employers introduced these new open-ended HMOs in hopes of reducing their runaway medical costs. In the past, young, healthy employees tended to join HMOs while their illness-prone, older colleagues stuck with family physicians and indemnity plans that cost companies more. But don't let corporate economy deprive you of getting the health care that suits you best. ''Having the option of seeing an outside specialist occasionally can be good security,'' advises Cynthia Hosay, senior vice president at Martin E. Segal, an employee-benefits consulting firm in New York City. ''But if you plan to use an outside doctor fairly frequently, you are better off in a traditional health plan.'' -- P.W.