CONTEXT: Preventive services are not delivered at optimal rates in primary care settings, and the literature suggests that a systems approach is key to improvement. Studying variation among clinics could help us to understand the extent of system use in practice. PRACTICE PATTERN EXAMINED: The proportion of patients who are up-to-date for preventive services in 44 primary care practices in the Midwest. PREVENTIVE SERVICES EXAMINED: Papanicolaou (Pap) smear, cholesterol testing, mammography, clinical breast examination, blood pressure measurement, influenza and pneumococcal vaccinations, and advice on tobacco use. DATA SOURCE: 6830 patients surveyed after their clinic visit (response rate, 85%). RESULTS: The proportion of patients up-to-date for preventive services varied widely among clinics. For example, up-to-date rates for Pap smear testing ranged from 70% to 93% and 45% to 88% for cholesterol screening. There was little correlation between a clinic's performance on one preventive service (relative to the other 43 clinics) and its performance on others. When correlations between pairs of up-to-date rates within clinics were examined, only 4 of 28 service pairs were positive and statistically significant and only 1 had a correlation coefficient that exceeded 0.5 (for mammography and clinical breast examination). CONCLUSION: There is wide variation in the rates at which various preventive services are performed, both between and within clinics. This variation, which is probably due to a lack of organized prevention systems that cover multiple services, provides a clear target for improvement efforts.