Two unrelated children with thrombotic disease associated with inherited protein S deficiency and osteopenia have been identified. Measurement of protein S yielded markedly reduced levels of free protein S (less than 12.5%) in both propositi, a normal level of total protein S (79%) in propositus #1, and markedly reduced level of total protein S (34%) in propositus #2. Bone densitometry measurements of the two children revealed trabecular vertebral bone mineral content below two standard deviations. This defect is associated with vertebral body compression fractures in propositus #2. Therefore, it is hypothesized that protein S deficiency is associated with abnormalities of bone mineral density.