The efficacy of prophylactic antibiotics for preventing infectious complications at the site of pit viper envenomation has not been well studied. We undertook a prospective, controlled trial of antibiotic treatment versus no antibiotic treatment among 114 victims of crotalid envenomation in Ecuador's Amazon rain forest. A group of 59 patients received intravenous gentamicin and chloramphenicol, and 55 patients did not. All other aspects of care were identical. There were no statistically significant differences between antibiotic-treated and untreated patients with regard to demographics, delay in treatment, clinical and laboratory evidence of severity of envenomation, or use of antivenin. Nine abscesses occurred, six in the antibiotic-treated group and three in the untreated group. The results of this study did not show any statistically significant differences in outcome in terms of the number of abscesses that occurred between antibiotic-treated and untreated patients. Based on this lack of differences, routine use of prophylactic antibiotics for prevention of infectious complications of crotalid envenomation cannot be recommended.