Thrombocytopenia associated with the presence of a heparin-dependent platelet aggregating factor developed in two patients after hemodialysis with heparin. It resolved in one patient after heparin was stopped; but persisted in the other during a two-week heparin-free period and intermittently thereafter. We suggest that when heparin causes thrombocytopenia in dialysis patients the heparin should be stopped whenever possible, but this may not be necessary in all patients.