Formosan Russell's viper (Daboia russelli siamensis) is the sixth most frequent cause of snakebite in Taiwan. Its venom has been thought to have both neurotoxic and hematoxic properties. This viper's snakebite is rare and thus scarcely subjected to systemic studies. In this paper, we retrospectively analyzed and described 18 cases of viper snakebite from 1987 to 1999. Like that of the Russell's viper snakebite in other South East Asian areas, varied degrees of acute renal failure, incoagulable blood with bleeding diathesis and hemolysis were the major symptoms found in the systemic envenoming patients. Systemic thrombosis seems to be the distinguishing feature in Formosan Russell's viper snakebite. Neither symptoms nor signs of neuromuscular junction blocking effects were observed, which is another difference from symptoms observed after bites of some other Russell's viper subspecies, suggesting a significant geographic variation. These findings confirmed the clinical importance of Russell's viper snakebite in Taiwan.