We developed a computer-stored medical record system containing a limited set of the total clinical data base—primarily diagnostic studies and treatments. This system responds to its own content according to physician-authored reminder rules. To determine the effect of the reminder messages generated by 1490 rules on physician behavior, we randomly assigned practitioners in a general medicine clinic to study or control groups. The computer found indications for six different actions per patient in 12 467 patients during a 2-year study: 61 study group residents who received computer reminders responded to 49% of these indications; 54 control group residents, to only 29% (p < 0.0001). Preventive care (occult blood testing, mammographic screening, weight reduction diets, influenza and pneumococcal vaccines) was affected. The intentions of the study group to use a given action for an indication predicted their response to the indications (p < 0.03, r2 = 0.33). The intentions of the control residents did not.