Objectives In 30% of patients with primary biliary cirrhosis (PBC) ursodeoxycholic acid (UDCA) causes full biochemical normalization, while 70% are incomplete responders. The only differences between the two groups are the significantly higher cholestasis indices in the incomplete responders. In these patients we investigated whether the strongly choleretic sulindac together with UDCA is superior to UDCA monotherapy. Design and methods Twenty-three patients with PBC incompletely responding to UDCA monotherapy were entered in the open label study for 12 months. Eleven patients (stage II, seven; III, two; and IV, two) received UDCA (10-15 mg/kg/day) plus sulindac (100-300 mg/day) (Group 1). Twelve patients (stage 1, six; 11, four; III, one; and IV, one) were treated with UDCA alone (Group 11). Liver biochemistry, analysis of antimitochondrial, antinuclear, smooth muscle, and liver-kidney-microsomal antibodies, ultrasonography and gastroscopy were done in regular intervals. Results In Group I all liver indices, IgG, IgM and IgA significantly improved although pretreatment data and stages of the disease tended to be higher than in Group II. In five patients of Group I liver histology improved slightly. Sulindac was well tolerated. The biochemical indices did not further improve on UDCA monotherapy. Conclusions Sulindac in combination with UDCA further improves liver biochemistries in patients with PBC who responded incompletely to UDCA alone.