The occurrence of peptic ulcer is characterized by a seasonal variation, the meaning of which is poorly understood. The present study examined the periodicity of hospital admissions and mortality resulting from gastric and duodenal ulcer to determine its etiologic relevance. Ulcer periodicity was studied in the nationwide data sets of the Department of Veterans Affairs, the Health Care Financing Administration, and the Vital Statistics. Both ulcer types were characterized by similar patterns of periodicity. Hospital admissions peaked during the first 3 months of the year, followed by a marked decline during summer and a second smaller peak around October. This pattern occurred independently of age, sex, race, and place of residence. It also pertained to ulcers complicated by hemorrhage or perforation. Although total admissions peaked earlier during the year and showed a less consistent peak in October, there was a close resemblance between the periodicity of all diseases and that of peptic ulcer. Mortality was highest in January and lowest in July for all disease and ulcer diseases alike. The similarity between the periodicity of peptic ulcer and other diseases suggests that the major factors responsible for the cyclic behavior of gastric and duodenal ulcer are not particular to these diseases but affect other unrelated diseases alike.