Twenty-nine clinically well-characterized, symptomatic index patients, 15 with protein C and 14 with protein S deficiency, in whom the genetic defect had been identified, were investigated for the presence of factor V Leiden. In six of 15 (40%) propositi with protein C and four of 14 (29%) with protein S deficiency, factor V Leiden was present. The age at first thrombosis was significantly lower (P < 0.001) in the ten propositi with a combined genetic defect (mean age 18.4 × 6.6 years) than in those with a single defect (mean age 32.6 × 10.4 years). Spontaneous occurrence, recurrence and site of thrombosis were similar in propositi with the single and the combined defect. Family studies led to the identification of a combined defect in 18 individuals from 11 families (11 propositi and 29 relatives), seven subjects had no abnormality, and in 15 a single defect was found. In individuals with a combined defect, thrombosis-free survival time was significantly shorter than in individuals with a single defect, even after exclusion of index patients. None of the seven individuals without genetic abnormality had experienced thrombosis. Our findings indicate a higher risk for development of thrombosis in individuals with a combined defect compared with those with a single defect. Blood Coag Fibrinol 9:85–89 × 1998 Rapid Science Ltd.