Based on the US Preventive Services Task Force recommendations, we studied how health insurance type, ie, fee-for-service (FFS) or health maintenance organization (HMO), affects the utilization of preventive services of differing effectiveness.Household survey data from the 1993 and 1994 National Health Interview Surveys.We compared the use of mammograms, Pap smears, blood pressure measurements, counseling about hormone replacement therapy (HRT), and general physical examinations in FFS plans and HMOs. We used the bivariate probit model to control for selection bias caused by the unobservable factors in the choice of health insurance type.Enrollees in HMOs obtained more Pap smears, blood pressure measurements, mammograms (women 40 to 49 years old), and general physical examinations than enrollees in FFS plans. No significant difference was found between FFS plans and HMOs for the use of mammograms (women aged 30 to 39 years and 50 to 64 years) or HRT counseling. The correlation ratios from bivariate probit estimations indicated no selection bias favoring HMOs; for some preventive services, selection bias favored FFS plans.Compared with enrollees in FFS plans, persons in HMOs used more preventive services, including less effective ones. Not controlling for selection bias underestimated the HMO effect.