We evaluated the effects of an appointment reminder system on the appointment-keeping behavior and blood pressure control status of hypertensive patients receiving care in an urban medical clinic. The study population consisted of 973 adult hypertensive patients receiving medical care at a family practice clinic affiliated with the Wayne State University School of Medicine, Detroit, Michigan. Of the 973 patients enrolled in the study, 486 were randomly assigned to a group that received appointment reminder cards and telephone calls to reschedule missed appointments, and 487 were assigned to a nonintervention control group. Patients were followed-up with regard to their appointment-keeping behavior and blood pressure control status, five to eight months after being entered in the study. The study results indicate that patients in the appointment reminder group kept significantly more appointments and were less likely to drop out of treatment than patients in the control group. The dropout rate was 46 percent lower among patients in the appointment reminder group than in the control group. Patients in the appointment reminder group also demonstrated slightly better blood pressure control at the end of the study compared with patients in the control group, although none of the differences in blood pressure levels between groups were statistically significant at the 5 percent level.