CONTEXT: Although there has been enormous interest in continuous quality improvement (CQI) as a measure to improve health care, this enthusiasm is based largely on its apparent success in business rather than formal evaluations in health care. OBJECTIVE: To determine whether a managed care organization can increase delivery of eight clinical preventive services by using CQI. DESIGN: Primary care clinics were randomly assigned to improve delivery of preventive services with CQI (intervention group) or to provide usual care (control group). INTERVENTION: Through leadership support, training, consulting, and networking, each intervention clinic was assisted to use CQI multidisciplinary teams to develop and implement systems for delivery of preventive services. SETTING: 44 primary care clinics in greater Minneapolis-St. Paul. PATIENTS: Patients 19 years of age and older completed surveys at baseline (n = 6830) and at follow-up (n = 6431). Medical chart audits were completed on 4777 patients at baseline and 4546 patients at follow-up. MAIN OUTCOME MEASURES: The proportion of patients who were up-to-date (according to chart audit) and the proportion of patients who were offered a service if not up-to-date (according to patient report) for 8 preventive services. RESULTS: Compared with the control group, based on the proportion of patients who were up-to-date, use of only one preventive service--pneumococcal vaccine--increased significantly in the intervention group (17.2% absolute increase from baseline to follow-up compared with a 0.3% absolute increase in the control group, P = 0.003). Similarly, based on patient report of being offered a service if not up-to-date, delivery of only one preventive service--cholesterol testing--significantly increased in the intervention group compared with the control group (4.6% increase vs. 0.4% absolute decrease in the control group; P = 0.006). CONCLUSION: In this trial, CQI methods did not result in clinically important increases in preventive service delivery rates.