The aim of this study was to compare the duodenal ulcer healing effects of morning (08.00 hours) vs. single bedtime (22.00 hours) doses of 40 mg famotidine, bearing in mind that the known efficacy of bedtime doses of H2-antagonists is regarded as evidence of the predominance of nocturnal gastric acidity in the pathogenesis of duodenal ulcer. This randomized double-blind multicentre trial was conducted in a total of 127 patients with endoscopically proven active duodenal ulcer. Nine patients dropped out and thus 118 were included in the final analysis. The duration of treatment was 4 weeks, and this was extended to 8 weeks in patients whose ulcers failed to heal by week 4. The patients in the two treatment groups were well matched for age and sex. The therapeutic efficacy parameters were endoscopic healing of the ulcer lesion and disappearance of pain. Results were compared using the chi-square method. The 4- and 8-week (cumulative) ulcer healing rates in the patients treated with the morning dose of famotidine were 77.2% and 86%, respectively, compared with 78.6% and 91.8% in those who received the bedtime dose. The differences failed to prove statistically significant either at week 4 (P = 0.85) or at week 8 (P = 0.31). The percentages of patients with ulcer pain, evaluated weekly, were similar in the two treatment groups. The equivalent efficacy of the morning and bedtime famotidine regimens raises doubts concerning the predominance of nocturnal gastric acidity in the pathogenesis of duodenal ulcer.