Two combinations of lisinopril and hydrochlorothiazide (L/HCTZ, 20 mg/12.5 mg and 20 mg/25 mg) are currently marketed. The licences are based primarily on a dose ratio study and a parallel titration study. In the former randomized study of 209 patients with sitting DBP of 90-115 mm Hg, the antihypertensive effects of L/HCTZ 20 mg/12.5 mg and 20 mg/25 mg were significantly greater than either lisinopril or hydrochlorothiazide monotherapy (P less than or equal to 0.01), while the effect of L/HCTZ 20 mg/6.25 mg, was greater than hydrochlorothiazide but not lisinopril monotherapy. In the latter study, 394 patients with sitting DBP of 90-120 mm Hg received lisinopril 20 mg, HCTZ 12.5 mg, or L/HCTZ 20 mg/12.5 mg once daily for 12 weeks of double-blind therapy. All of the dose regimens could be doubled at 4 and 8 weeks for adequate BP control. The L/HCTZ group had significantly greater effect than either monotherapy (P less than or equal to 0.01). Thus, concomitant therapy with lisinopril and hydrochlorothiazide provides BP reduction beyond that of either monocomponent. Studies on new combinations of lisinopril and hydrochlorothiazide are under way to increase prescribing options for practitioners.