Conflicting data have been reported on the risk for venous thrombosis in subjects with low free protein S levels. We performed a post-hoc analysis in a single-center retrospective thrombophilic family cohort, to define the optimal free protein S level that can identify subjects at risk for venous thrombosis. Relatives (1143) were analyzed. Relatives with venous thrombosis (mean age 39 years) had lower free protein S levels than relatives without venous thrombosis (P < .001), which was most pronounced in the lowest quartile. Only relatives with free protein S levels less than the 5th percentile (< 41 IU/dL) or less than the 2.5th percentile (< 33 IU/dL) were at higher risk of first venous thrombosis compared with the upper quartile (> 91 IU/dL); annual incidence 1.20% (95% confidence interval [CI], 0.72-1.87) and 1.81% (95% CI, 1.01-2.99), respectively; adjusted hazard ratios 5.6, (95% CI, 2.7-11.5) and 11.3 (95% CI, 5.4-23.6). Recurrence rates were 12.12% (95 CI, 5.23-23.88) and 12.73% (95% CI, 5.12-26.22) per year; adjusted hazard ratios were 3.0 (95% CI, 1.03-8.5) and 3.4 (95% CI, 1.1-10.3). In conclusion, free protein S level can identify young subjects at risk for venous thrombosis in thrombophilic families, although the cutoff level lies far below the normal range in healthy volunteers.