A family with a platelet release abnormality (PRA) is described. The only son also showed a reduced rate of platelet aggregation in response to ristocetin, markedly reduced levels of von Willebrand's factor (vWf, ristocetin cofactor), and increased mobility of factor VIII-like antigen, features which were suggestive of von Willebrand's disease (vWd). No inhibition of vWf was found in his plasma. Family studies showed no evidence of vWd in the mother. The father's investigations showed a low rate of ristocetin aggregation on one of the two occasions when it was tested and low vWf on two of four occasions. Despite repeated testing, the findings in the father did not conclusively rule out the possibility of mild vWd, and it was impossible to determine whether the vWd in the son was inherited or arose as a mutation. The findings in this family suggest a possible relationship between abnormalities of the factor VIII complex and defective platelet function.