A possible influence of the aromatic retinoid acitretin on the hypothrombinemic action of phenprocoumon was investigated in 10 healthy male volunteers, 22-33 years of age. In a run-in period of 17 days the prothrombin complex activity (Quick) of the subjects was adjusted by individual doses of phenprocoumon (range 1.5-3 mg/day) to a therapeutic level of around 20% (INR = 3.6). Afterwards the dosage regimens were kept unchanged until day 40 when 60 mg oral vitamin K1 were administered. From day 18 to day 28 once daily doses of 50 mg acitretin were applied concurrently with phenprocoumon. The mean (+/- SD) prothrombin complex activity increased slightly and steadily from 19 +/- 1% on day 10 over 22 +/- 1% on day 18 to 24 +/- 4% on day 28 and reached 27 +/- 4% on day 38 (the corresponding INRs were 3.30, 2.91, 2.71 and 2.46 respectively). There were, however, no relevant changes of the Quick values in any subject which could be attributed to acitretin. It can be concluded that, under the present experimental conditions, acitretin did not affect significantly the anticoagulant action of phenprocoumon.