Abnormality of humoral and cellular immune functions and the association of autoimmune diseases are frequently observed in primary biliary cirrhosis (PBC). The prevalence of autoimmune diseases was studied in 97 Japanese patients with PBC. Sjögren's syndrome was diagnosed in 33 percent of these patients, arthritis in 22 percent, scleroderma in 11 percent, CREST syndrome in 4 percent, Raynaud's phenomenon in 8 percent, autoimmune thyroiditis in 3 percent, respectively. Fifty-five percent of the patients had at least one autoimmune disease and 19 percent had two or more such disorders. In this study, the prevalence of associated autoimmune diseases was somewhat low compared to that of European and American studies. Geographical variations, however, might exist in the prevalence of autoimmune associations, and the frequent occurrence of coexisting autoimmune diseases suggests an autoimmune pathogenesis in PBC.