A 42 year old man, treated for phlebitis with subcutaneous heparin, developed vascular occlusion in both legs with thrombocytopaenia. After surgery, heparin was discontinued and replaced by low molecular weight heparin (CY 216 Choay) without any success. Both commercial heparin and CY 216 Choay produced platelet aggregation in vitro. Heparin was stopped and replaced by dextran 40,000 with anti-aggregating drugs (flurbiprofen) followed by antivitamin K drugs. Thrombocytopaenia resolved three days later and the patient was discharged without sequelae. It therefore appeared necessary to carry out aggregation tests both with heparin and low molecular weight heparin if thrombocytopaenia occurs whilst heparin is being used.