We analyzed patterns of receipt of preventive services among middle-aged women, with particular attention to health insurance coverage, based on data from the National Health Interview Survey. Lack of insurance was most prevalent among socioeconomically disadvantaged women at high risk for disease and was the strongest predictor of failure to receive screening tests. The relative risk of inadequate screening for uninsured compared with insured women was 1.60 (95% confidence interval [Cl], 1.40 to 1.83) for blood pressure checkups, 1.55 (95% Cl, 1.43 to 1.68) for cervical smears, 1.52 (95% Cl, 1.41 to 1.63) for glaucoma testing, and 1.42 (95% Cl, 1.33 to 1.51) for clinical breast examination. Controlling for demographic and health status variables did not diminish the effect of insurance coverage. We conclude that inadequate insurance coverage leads to "reverse targeting" of preventive care--that is, populations at highest risk are least likely to be screened. This compromises both the effectiveness and the cost-effectiveness of screening.