The association of primary biliary cirrhosis (PBC) and autoimmune hepatitis (AIH) is thought to be rare, and its optimal treatment is unknown. Of 130 consecutive patients with a diagnosis of PBC, we identified 12 cases (9.2%) of overlap syndrome (10 females, 2 males; median age, 50 years) strictly defined by the presence of at least two of the three recognized biochemical, serological, and histological criteria of each disease. One patient had initially pure PBC and developed AIH characterized by a flare of alanine transaminase (ALT) (1,330 IU/L; N < 35), elevated immunoglobulin G (IgG) (42 g/L; N < 14.0), and presence of anti-smooth muscle antibodies (ASMA) after 20 months of ursodeoxycholic acid (UDCA) therapy. A complete clinical and biochemical remission was achieved under combination of corticosteroids and UDCA. Eleven patients had features of both diseases at presentation: high serum levels of alkaline phosphatase (AP) (median: 280 IU/L; N < 100), ALT (140 IU/L), and IgG (30.8 g/L), presence of mitochondrial antibodies (n = 9) or ASMA (n = 9), florid bile duct lesions (n = 8), and moderate or severe periportal or periseptal lymphocytic piecemeal necrosis (n = 11). UDCA (13-15 mg/kg/d) given alone in 5 patients induced a significant decrease in biochemical cholestasis but not in ALT levels, and liver fibrosis progressed in 3 patients. Corticosteroids given alone in 6 patients induced a significant decrease in ALT, IgG, and AP levels, but none had a biochemical normalization. The patients with persistently abnormal liver tests under either UDCA or corticosteroids received both UDCA and corticosteroids. A further marked biochemical improvement was observed, and all patients became asymptomatic. We conclude that, in patients with PBC: 1) overlap syndrome with AIH is not rare; 2) flares of AIH may occur either spontaneously or under UDCA; and 3) combination of UDCA and corticosteroids is required in most patients to obtain a complete biochemical response. Overlap syndrome may represent an important and unrecognized cause of resistance to UDCA in patients with PBC.