Severe heparin-induced thrombocytopaenia associated with thromboembolism is a well known complication, although the exact pathogenic mechanism remains unclear. It sets the problem of whether to continue heparin therapy because standard heparin must be withdrawn. Heparin is a mucopolysaccharide composed of fractions of different molecular weights. The fractions with high molecular weights have been held responsible for these severe thrombocytopenias and so, the use of low molecular weight heparin has been suggested. The authors used subcutaneous low molecular weight heparin (CY 216 Choay Institute) at empirical doses of 350 to 1 500 units/kg/24 hour in six cases of severe heparin-induced thrombocytopaenia. Platelet counts rapidly returned to normal (4 days on average) in 5 cases. Thrombocytopaenia persisted with low molecular weight heparin in 1 case. The study of platelet aggregation was positive with low molecular weight heparin in this case and the platelet count returned to normal when the treatment was withdrawn. The authors conclude that, although low molecular weight heparin is useful in severe heparin-induced thrombocytopaenia, its efficacy remains modest. Not only may platelet aggregation persist with low molecular weight heparin which rekindles the debate as to its pathogenic mechanism, but also low molecular weight heparin may have a slight antithrombin effect which limits its use in patients at high risk of thromboembolism, imposing treatment with fast acting vitamin K antagonists.