The clinical, histologic, and immunofluorescent findings in 86 cases of pyoderma gangrenosum seen at the Mayo Clinic between 1970 and 1983 were reviewed. Males and females were affected in equal numbers. The most frequent site of lesions was the leg. Sixty-seven patients (78 per cent) had associated systemic disease, with arthritis and inflammatory bowel disease being commonest. Cutaneous histopathologic changes varied with the site of biopsy. Lymphocytic vasculitis was predominant in the zone of erythema peripheral to the area of ulceration, while neutrophilic infiltrate and abscess formation were more prominent centrally. In most cases studied, direct immunofluorescence showed immunoglobulins and complement deposited in and around superficial and deep dermal vessels.