AIM: A circannual variation in the onset of several acute diseases,mostly dealing with cardiovascular system, has been reported.The present study was to verify the possible existence of a seasonal variability in the onset of acute pancreatitis.METHODS: All patients consecutively admitted to the Hospital of Ferrara, Italy, between January 1998 to December 2002,whose discharge diagnosis was acute pancreatitis, were considered. According to the time of admission, cases were categorized into twelve 1-mo intervals and in four periods by season. x2 test for goodness of fit and partial Fourier series were used for statistical analysis.RESULTS: During the study period, 549 cases of acute pancreatitis were observed. A significant peak of higher incidence was found in March-May, both for total population,males and subgroups with and without cholelithiasis or alcoholism. Fourier analysis showed the existence of a circannual rhythmic patt with its main peak in March (95%C.L.: February-April, P = 0.005), and a secondary one in September. Death occurred more frequently in DecemberFebruary, compared to the other periods (P = 0.029), and chronobiologic analysis yielded a seasonal peak in NovemberDecember (P＜0.001).CONCLUSION: This study shows the existence of a circannual variation in the onset of acute pancreatitis, with a significanty higher frequency of events in the spring, especially for patients with cholelithiasis or alcoholism. Moreover, events occurring during the colder months seem to be characterized by a higher mortality rate.