The relative effectiveness of the diverse approaches used to promote preventive care activities, such as cancer screening and adult immunization, is unknown. Despite many high-quality published studies, practices and policymakers attempting to improve preventive care have little definitive information on which to base decisions. Thus, we quantitatively assessed the relative effectiveness of previously studied approaches for improving adherence to adult immunization and cancer screening guidelines. MEDLINE, the Cochrane Effective Practice and Organization of Care Review Group register, previous systematic reviews, and the Medicare Health Care Quality Improvement Project database. Controlled clinical trials that assessed interventions to increase use of immunizations for influenza and pneumococcal pneumonia and screening for colon, breast, and cervical cancer in adults. Two reviewers independently extracted data on characteristics and outcomes from unmasked articles. Intervention components to increase use of services were classified as reminder, feedback, education, financial incentive, legislative action, organizational change, or mass media campaign. Of 552 abstracts and articles, 108 met the inclusion criteria. To assess the effect of intervention components, meta-regression models were developed for immunizations and each cancer screening service by using 81 studies with a usual care or control group. The most potent intervention types involved organizational change (the adjusted odds ratios for increased use of services from organizational change ranged from 2.47 to 17.6). Organizational change interventions included the use of separate clinics devoted to prevention, use of a planned care visit for prevention, or designation of nonphysician staff to do specific prevention activities. The next most effective intervention components were patient financial incentives (adjusted odds ratios, 1.82 to 3.42) and patient reminders (adjusted odds ratios, 1.74 to 2.75); the adjusted odds ratios ranged from 1.29 to 1.53 for patient education and from 1.10 to 1.76 for feedback. Rates of adult immunization and cancer screening are most likely to improve when a health care organization supports performance of these activities through organizational changes in staffing and clinical procedures. Involving patients in self-management through patient financial incentives and reminders is also likely to positively affect performance.