Autoimmune hepatitis (AIH)-primary biliary cirrhosis (PBC) overlap syndrome (OS) is a vaguely defined entity demonstrating features of AIH and PBC. We investigated the usefulness of IgG and IgM immunostaining for the distinction of AIH and PBC and their staining pattern in cases of possible OS. The approximate quantity of IgG+ and IgM+ periportal inflammatory cells in immunohistochemical analysis were compared in cases of AIH, PBC, and OS. AIH cases showed predominant IgG immunostaining of periportal inflammatory cells. A significant number of PBC cases also demonstrated IgG predominance rather than IgM. Six OS cases had IgG predominance, 4 had IgM predominance, and 1 was equivocal. The usefulness of IgG and IgM immunostaining is limited in PBC cases with IgM predominance for excluding AIH. IgG predominance is not specific for AIH. OS does not demonstrate either IgG or IgM predominance (P > .2) and does not help classify OS into either category.