Duodenal ulcer is a recurrent disease with seasonal periodicity for pain and complications such as hemorrhage and perforation. Ulcer craters or symptoms seem to occur preferentially in early spring and autumn. Since acid secretion is one of the pathogenetic factors of the disease, we analyzed retrospectively basal and maximal (pentagastrin) acid secretion data obtained in 341 consecutive patients according to the month in which they were obtained. The patients were classified according to the activity of their ulcer (active, non active) and to the level of the peak acid secretion (hypersecretors, normosecretors). Basal acid concentration and output, and peak acid output were, both overall and month by month, higher in patients with active duodenal ulcer disease than in those who were non active, and in hypersecretors than in normosecretors. For all 341 patients as well as for normosecretors and non active ulcer patients, a triannual rhythm was detected for stimulated acid concentration and peak acid output. The highest values were noted in February, June, and October (period: 4 months). The amplitude of these rhythms was 3 to 4 percent, with differences between highest and lowest values of 30.4 mmol/l for concentration and 17.1 mmol/h for peak acid output. These rhythms for acid secretion during the year may contribute to the periodicity of duodenal ulcer events and should be analyzed in association with other factors which could be implied in ulcerogenesis. Moreover, this seasonal periodicity of acid secretion in duodenal ulcer should be taken into account in all therapeutic trials in which acid secretion is analyzed.