The effects of self-monitoring of blood pressure on the control of hypertension were examined in this study. Failure of patients to comply with treatment is presumably attributable in part to the fact that hypertension usually is asymptomatic until complications develop. Self-monitoring might make visible an otherwise asymptomatic condition, and thereby increase motivation. One hundred hypertensive patients beginning outpatient treatment were randomly assigned to experimental and control groups. Subjects were given a sphygmomanometer and instructed in its use. Both groups were given similar antihypertensive medications. After six months of treatment, mean systolic pressure was significantly lower (11.4 mm Hg, p smaller than 0.05) in the experimental than in the control subjects. However, the mean baseline systolic pressure in the control group was 3.9 mm Hg less than that of the experimental group. If this value is substracted from the difference between the last mean systolic pressures in the two groups, the 7.5 mm Hg difference is seen as a very modest effect of self-monitoring. Diastolic blood pressure was insignificantly lower for experimental subjects. Compared to the potent effectiveness of drugs in reducing blood pressure, self-monitoring was of little value.