Two children aged 3 months and 14 years developed heparin-induced thrombocytopenia whilst receiving heparin. The clinical course was complicated by renal vein thrombosis in the 3-month-old infant and subcutaneous haemorrhage requiring resuscitation in the 14-year-old child. Anticoagulant therapy was discontinued immediately in the 3-month-old child. In the 14-year-old child, because of the need for continued anticoagulation therapy, low molecular weight heparin was used until warfarin could be substituted.