HLA A and B antigens in 46 patients with chronic pancreatitis were studied in Japan. Patients were divided into the two groups according to the etiology of the disease: one was chronic idiopathic pancreatitis (18 cases) and the other was chronic alcoholic pancreatitis (28 cases). One hundred twenty unrelated subjects were also examined as control. HLA B5 was detected in 14 of 18 cases of chronic idiopathic pancreatitis, which was significantly higher statistically than in control (P less than 0.001, corrected P less than 0.05). In contrast, no HLA antigens of locus A and B were found which had a relationship to chronic alcoholic pancreatitis. These results suggest that some genetic factors may be implicated in the development of chronic idiopathic pancreatitis and differing from that of alcoholic pancreatitis.