Thrombosis is a serious medical problem causing considerable suffering and occasional death. Thrombotic events can occur in pediatric patients, and certain genetic risk factors can be involved in the pathogenesis. Children who inherit these genetic defects may be predisposed to unexpected morbid clotting events even as early as the perinatal period. The specific aims of this study are 1) to assess a retrospectively identified cohort of children with thrombosis for an underlying genetic predisposition for future adverse clotting; 2) to examine the prevalence rates of pediatric thrombosis due to an underlying genetic cause; and 3) to investigate the prevalence rates of genetic factors in children with and without recurrent thrombosis. The study will involve identification of subjects with thrombotic events at age 12 years or younger, with visits for family history and blood drawing. The blood will be tested for 6 genetic defects known to be associated with thrombosis. Followup genetic and hematologic counseling will be offered if necessary.