To assess the impact of prompting physicians on health maintenance, answer questions regarding the mode of delivery, and identify opportunities and limitations of this information intervention. Systematic electronic and manual searches (January 1, 1966, to December 31, 1996) were conducted to identify clinical trial reports on prompting clinicians. Three eligibility criteria were applied: (1) randomized controlled clinical trial, (2) clinician prompt, alert, or reminder in the study group and no similar intervention in the control group, and (3) measurement of the intervention effect on the frequency of preventive care procedures. Data were abstracted by independent reviewers using a standardized abstraction form, and quality of methodology was scored. A series of meta-analyses on triggering clinical actions was performed using the random-effects method. The statistical analyses included 33 eligible studies, which involved 1547 clinicians and 54 693 patients. Overall, prompting can significantly increase preventive care performance by 13.1% (95% confidence interval [CI], 10.5%-15.6%). However, the effect ranges from 5.8% (95% CI, 1.5%-10.1%) for Papanicolaou smear to 18.3% (95% CI, 11.6%-25.1%) for influenza vaccination. The effect is not cumulative, and the length of intervention period did not show correlation with effect size (R = -0.015, P = .47). Academic affiliation, ratio of residents, and technique of delivery did not have a significant impact on the clinical effect of prompting. Dependable performance improvement in preventive care can be accomplished through prompting physicians. Vigorous application of this simple and effective information intervention could save thousands of lives annually. Health care organizations could effectively use prompts, alerts, or reminders to provide information to clinicians when patient care decisions are made.