1. Thirty-five patients on continuous therapy with warfarin were given omeprazole 20 mg once daily and placebo each for 3 weeks according to a two-centre randomised double-blind cross-over design. 2. Blood samples were obtained once weekly during the run-in and follow-up periods as well as during the first 2 weeks of each treatment period, and twice during the last week of each treatment period. Plasma concentrations of R- and S-warfarin were measured by h.p.l.c. and the anticoagulant effect was assessed using the Trombotest. 3. Twenty-eight patients were evaluated. The mean plasma concentration of R-warfarin was increased by 9.5% during omeprazole treatment compared with placebo, while that of S-warfarin, the more active isomer, was unaffected. The coagulation time was not significantly changed (106 s during omeprazole and 98 s during placebo). Corresponding TT-values (Trombotest) were 8.8 and 9.9 (NS).