Necrotizing fasciitis is a rare, rapidly progressing soft-tissue infection. Early clinical recognition is difficult, and there is often a fatal delay in appropriate treatment. We evaluated the use of frozen-section biopsies for the rapid diagnosis of this disease in its early stages. Nineteen cases of necrotizing fasciitis were studied over a 13-year period. In all eight cases in which frozen-section biopsy was performed within zero to four days (average, 21 hours) after the onset of symptoms, histologic study confirmed the diagnosis, and surgical d ebridement was immediately performed. Only one patient died. In 11 patients, the diagnosis was made on clinical grounds, and surgical therapy was delayed for 1 to 15 days (average, 6) after the onset of symptoms because of failure to recognize the disorder immediately. Eight patients died (P less than 0.02). Our study indicates that rapid performance of frozen-section soft-tissue biopsy early in the evolution of a suspect lesion may provide a definitive and life-saving diagnosis.