Carvedilol [25 mg once daily] (o.d.) was compared to atenolol (50 mg o.d.) as an adjunct to pre-existing hydrochlorothiazide (HCTZ) monotherapy in patients with mild to moderate hypertension [diastolic blood pressure (DBP), 100-115 mm Hg]. After a placebo run-in phase of 2 weeks, 131 patients received 25 mg HCTZ o.d. for 4 weeks. In all, 122 patients were transferred to the double-blind phase, in which 25 mg carvedilol or 50 mg atenolol was randomly added to HCTZ. After an additional 6 weeks of treatment, 112 patients were evaluable for efficacy (C/HCTZ group, n = 54; A/HCTZ group, n = 58). Blood pressure was measured and the heart rate was counted before medication, at 2-week intervals throughout the trial, and 2 h after medication on the 1st and the last day of the combination treatment period. Serum lipids were measured in addition to routine laboratory variables. A therapeutic response was defined as a reduction in supine and standing diastolic blood pressure to values of less than 90 mm Hg. In a relatively low number of patients (6 of 131), a response as defined above was achieved with HCTZ alone. This may be accounted for by the fact that patients were required to have a diastolic blood pressure of at least 100 mg Hg and by the relatively short period of monotherapy. The two groups of patients receiving different combination treatments were well matched for demographic data and blood pressure values before the adjunct was added. In both groups there was a marked additional blood pressure decrease on the initiation of combined treatment.(ABSTRACT TRUNCATED AT 250 WORDS)