Abnormalities of platelet function can result in defective hemostasis and a prolonged bleeding time in spite of a normal platelet count. Platelet function tests including platelet adhesiveness, platelet aggregation to adenosine diphosphate (ADP), collagen, and thrombin, clot retraction, and platelet factor 3 (PF3) release were performed in 16 patients who had mild to moderately severe bleeding diathesis. Hereditary platelet functional diseases which are reported include von Willebrand's disease, ADP release defect, PF3 release defect, and Glanzmann's thrombasthenia. Platelet dysfunction is described in association with the following diseases: connective tissue disorders, glycogen storage diseases, biliary cirrhosis, disseminated lupus erythematosus, thalassemia major, streptococcal infection, intravascular coagulation, and acyanotic congenital heart disease. A tentative classification of hereditary and acquired disorders of platelet function is presented.