OBJECTIVE: To compare the effectiveness of three computerized reminder systems in the delivery of five preventive procedures in family practice. DESIGN: Prospective, randomized, controlled study. SETTING: Ottawa Civic Hospital Family Medicine Centre. PARTICIPANTS: Of 8502 patients 15 years of age or more who were not in a hospital or institution 5883 were randomly assigned, by family, to a control group, a physician reminder group (passive) or a telephone or letter reminder group (active). The remaining 2619 patients were not included in the randomized portion of the study but were monitored. INTERVENTION: During 1 year the patients in the active reminder groups received a telephone call or letter reminding them of any overdue preventive procedures; for those in the passive reminder group the physician was reminded at an office visit to provide any overdue service. OUTCOME MEASURE: Rates of completion of the preventive procedures required. MAIN RESULTS: All three reminder systems significantly improved the delivery of preventive services (p less than 0.001). The procedure completion rates were 42.0% in the letter reminder group, 42.0% in the telephone reminder group, 33.7% in the physician reminder group and 14.1% in the randomized control group. The use of a letter was more cost-effective than the telephone system, but the physician reminder system was the most cost-effective. CONCLUSION: Computerized reminder systems do improve the delivery of preventive services in family practice.