An increase in factor VII coagulant activity is known to be an important risk factor for ischaemic heart disease. Four hundred and eight healthy male Post Office workers were screened in an occupational survey. Sixty eight (16.5%) of these had values of factor VII coagulant activity greater than 1.0 SD above the age related mean. A randomised double-blind crossover study was undertaken to investigate the effect of a fixed daily minidose of warfarin (1 mg) on the high activities of factor VII in these men. Forty two agreed to enter the study and 40 completed it. Their mean factor VII coagulant activity before warfarin treatment was 135.9%. Treatment with a fixed minidose of warfarin significantly reduced factor VII coagulant activity to 124.6%; there was no change on placebo. The prothrombin time was also significantly prolonged on active treatment although all the results remained within the normal range. These findings suggested a fixed minidose warfarin regime might be useful in the primary prevention of ischaemic heart disease by reducing high activities of factor VII.