We investigated the frequencies of coagulation factor deficiencies in a Japanese population. We measured factor II, V, VII and X activity in 100 healthy individuals. A specific factor deficiency was determined according to the factor activity and the ratio of the factor activity to that of other coagulation factors. Seven samples showed factor activity less than the mean -2SD of standardized factor activity (factor II: three; factor V: one; factor VII: one; factor X: one and factor V+factor VII: one). The samples with low factor II and factor VII activity were determined not to be due to deficiency because the ratios of these factor activities to other factor activities were within the range of the mean +/- 2SD. We measured activity ratios in the remaining 97 samples and identified one sample with factor V deficiency and two with factor VII deficiency. Thus, six samples with coagulation factor deficiency were identified (factor X: one; factor V: two; factor VII: two and factor V + factor VII: one). These results suggest that the Japanese population has relatively high frequencies of mild factor V, factor VII and factor X deficiencies, in which activity is reduced to approximately 50% (36-64%) of normal plasma.