Fifteen patients with definite hypothyroidism and two with probable hypothyroidism had extensive hemostatic profiles done. In 12 of the 16 patients tested, the platelet adhesiveness (platelet retention in a glass bead column) was abnormal, and in 1 more it was borderline. In the six patients who were studied repeatedly, hemostatic abnormalities either markedly improved or disappeared after treatment with L-thyroxine. It is concluded that, in addition to the previously reported coagulation factor deficiencies (which we also observed in some of our patients), low platelet adhesiveness occurs frequently in patients with hypothyroidism.