Gallstones from 82 patients were examined under a scanning electron microscope for evidence of bacteria, and the findings were compared with the clinical manifestations of the disease. Bacteria were present in 68% of pigment stones and the pigment portions of 80% of composite stones. These gallstones were referred to as infectious stones. No bacteria were found in cholesterol gallstones. Acute cholangitis was diagnosed in 52% of patients with infectious stones and in 18% of patients with noninfectious stones. Over half of the patients with noninfectious stones presented with mild symptoms. Infectious stones were more often associated with a previous common duct exploration, an urgent operation, infected bile, a common duct procedure, and complications. These data show that gallstone disease is more virulent in patients whose gallstones contain bacteria.