OBJECTIVE--To determine the risks of hospitalisation for bleeding peptic ulcer with the current prophylactic aspirin regimens of 300 mg daily or less. DESIGN--A case-control study with hospital and community controls. SETTING--Hospitals in Glasgow, Newcastle, Nottingham, Oxford, and Portsmouth. SUBJECTS--1121 patients with gastric or duodenal ulcer bleeding matched with hospital and community controls. RESULTS--144 (12.8%) cases had been regular users of aspirin (taken at least five days a week for at least the previous month) compared with 101 (9.0%) hospital and 77 (7.8%) community controls. Odds ratios were raised for all doses of aspirin taken, whether compared with hospital or community controls (compared with combined controls: 75 mg, 2.3 (95% confidence interval 1.2 to 4.4); 150 mg, 3.2 (1.7 to 6.5); 300 mg, 3.9 (2.5 to 6.3)). Results were not explained by confounding influences of age, sex, prior ulcer history or dyspepsia, or concurrent non-aspirin non-steroidal anti-inflammatory drug use. Risks seemed particularly high in patients who took non-aspirin non-steroidal anti-inflammatory drugs concurrently. CONCLUSION--No conventionally used prophylactic aspirin regimen seems free of the risk of peptic ulcer complications.