To further characterize inherited heterozygous protein S (PS) deficiencies, we studied 63 patients belonging to 33 families. Diagnosis of PS deficiency was based on protein S activity (PS Act) and/or free PS antigen (FPS Ag) levels below the lower limit of the normal range in patients not on oral anticoagulation. Depending on the level of total PS antigen (TPS Ag), two subpopulations could be distinguished: in the first one (25 patients belonging to 11 families) level of TPS Ag was reduced whereas in the second one (38 patients belonging to 22 families), TPS Ag was normal. In none of the families studied the two types of PS deficiency coexisted suggesting that they are different entities. In the 63 patients, thromboembolic events occurred in 57% of cases and were recurrent in 36.5% of patients. Age at the time of the first thrombosis ranged from 14 to 74 years, and was below 40 years in 69% of symptomatic cases. Thrombotic events were spontaneous in 64% of cases, and were associated with other risk factors in 36%. There was no apparent relationship between clinical status, symptomatic or asymptomatic, and the type or degree of the PS deficiency. Long-term anticoagulation prevented the recurrence of thrombosis in every case but one, and led to a decrease in circulating levels of C4b-binding protein suggesting the existence of a regulation between C4b-BP and PS concentrations. Together with previous reports, these findings underline the clinical and biological heterogeneity of inherited protein S deficiency.