Two randomized studies were designed to assess the safety, tolerability and efficacy of losartan 100 mg (L100) plus hydrochlorothiazide 12.5 mg (H12.5) in a single fixed-dose combination. In one study, subjects received losartan 50 mg (L50) plus H12.5 during an 8-week filter period. They were then randomized to either L100/H12.5 or L50/H12.5 for another 8 weeks, followed by L100/H12.5 for 44 weeks. The primary end point was safety of L100/H12.5 for 52 weeks. In the second study, subjects received L100 during an 8-week filter period. Subjects were then randomized to receive either L100/H12.5 or L100 for a further 8 weeks. The primary end point was change from baseline in sitting diastolic blood pressure (SiDBP) at week 8. Safety was assessed throughout both studies. L100/H12.5 reduced SiDBP and sitting systolic blood pressure (SiSBP) at 8 weeks, and when compared with L100, the differences were statistically significant for both measures (P<0.001). L100/H12.5 reductions SiDBP for 8 weeks were comparable to L50/H12.5. The efficacy of L100/H12.5 was maintained to week 52. Drug-related adverse events with an incidence ⩾ 2% in the L100/H12.5 group during the 52-week extension period were an increase in aspartate aminotransferase and in blood uric acid. Additionally, mean uric acid levels were reduced by 0.57 mg dl(-1) from baseline with long-term treatment with L100/H12.5 in subjects whose baseline uric acid level was >7.0 mg dl(-1). In conclusion, L100/H12.5 was shown to be more effective than L100 at reducing SiDBP and SiSBP and showed good tolerability in Japanese patients with essential hypertension.