A 1-year double-blind placebo-controlled study on the effects of diuretic withdrawal was conducted on a group of 62 previously well-controlled, mildly hypertensive patients. Data were collected on blood pressure (BP), biochemical laboratory values, and subjective reports of side-effects. Twenty-six percent of placebo subjects and 3% of the active treatment subjects reached preset criteria for the return of hypertension (reverters). The average systolic and diastolic pressures of all placebo-treated patients who did not revert showed statistically significant increases. BP control was quickly reestablished in reverters by restarting diuretic therapy. No substantial differences in side-effects were reported between the groups, and laboratory changes were those consistent with known metabolic effects of thiazide and thiazide-like diuretics. This study showed a much lower reversion rate after treatment withdrawal than previously reported by other investigators. It also showed significant increases in BP of placebo patients who did not revert. Long-term diuretic therapy retains its effectiveness in responsive mild hypertensive patients, potentially offering protection against the increased risks of mortality and morbidity associated with even slight elevations of BP. Withdrawal of diuretics cannot be recommended for patients with mild hypertension without use of other equally effective interventions to maintain optimum BP control.