Despite an emerging consensus on appropriate preventive services, a minority of patients receive them. A study was undertaken to assess the impact of computer-generated reminders to adult patients, their physicians, or both patients and physicians on adherence to five recommended preventive services: cholesterol measurements, fecal occult blood testing, mammography, Papanicolaou smears, and tetanus immunization. During the academic year 1988-1989, all 7397 adult patients and their 49 physicians in a university family medicine clinical practice were randomized by practice group into one of four study groups: control, physician reminders, patient reminders, and both physician and patient reminders. Adherence was defined in community-oriented terms: the percentage of patients within each group who had received the preventive service in the recommended interval. During the study period, adherence to four of the five preventive services increased significantly, with the largest increases in the physician and patient reminder group: cholesterol measurements increased from 19.5% to 38.1%, fecal occult blood testing 9.3% to 27.0%, mammography 11.4% to 27.1%, and tetanus immunization 23.4% to 35.4% (for each increase, P less than .0001, McNemar's chi-square test). In general, increases were greater in blacks and in patients with any form of insurance coverage. Computer-based physician and patient reminder systems have great promise of improving adherence to preventive services in primary care settings.