Envenomation by the Bothrops lanceolatus, a snake found only in Martinique, leads to swelling and pain, and occasionally to systemic signs and/or coagulopathy. Severe thromboses at some distance from the site of the bite may appear within 48 hr. Uncertainties as to the actual development of thrombotic complications in patients appearing to be suffering from moderate poisoning and as to the availability and the toxicity of a monospecific antivenom (AVS) initially led us to reserve antivenom for the most severe cases, and to use anticoagulants to prevent thromboses in all patients. This approach was modified after we observed serious thromboses in patients with moderate poisoning. Of 50 adult snake bite cases hospitalized between June 1991 and August 1994, 11 developed serious thrombotic complications at 36 /+- 27 hr (mean +/- SD) (range 12-96) following envenomation, despite early preventive anticoagulant therapy. Those included pulmonary embolism (two cases), cerebral infarction (six cases), myocardial infarction (one case), and cerebral and myocardial infarctions (two cases). Sixteen patients were not treated with AVS: 10 of these recovered without complications and six developed systemic thrombosis causing permanent disability in three cases. Thirty were treated with an intravenous infusion of 2-6 vials of AVS given 2-48 hr after the bite. Of these, three died of cerebral infarction that developed before the initiation of serotherapy. All others recovered. Among patients treated with AVS, three presented with mild anaphylactic reactions, while one developed serum sickness that responded to steroids. These data indicate that preventive anticoagulant therapy is of limited efficacy in Martinique.(ABSTRACT TRUNCATED AT 250 WORDS)