To elucidate sleep disturbances in the etiology of peptic ulcers, the prevalence of peptic ulcer disease was compared among shift workers and daytime workers. The subjects (N = 11,657) were employees of various institutions, such as factories, banks, or schools, and had undergone a mass x-ray examination of the upper gastrointestinal tract. With present shift workers (N = 2269), the prevalence of gastric ulcers was 2.38% (N = 54) and that of duodenal ulcers, 1.37% (N = 31). With the past shift workers (N = 2111), the prevalence of gastric ulcers was 1.52% (N = 32) and that of duodenal ulcers, 0.62% (N = 13). On the contrary, with the daytime workers (N = 6525) the prevalence of gastric ulcer was 1.03% (N = 67) and that of duodenal ulcer, 0.69% (N = 45). The working schedule for 752 examinees was unknown. The prevalence of gastric and duodenal ulcers was higher with shift workers than daytime workers. Thus, shift work involving sleep disturbances may play an important role in the development of peptic ulcers.