A 42-yr-old woman with long-standing ulcerative colitis of the descending colon, sigmoid, and rectum presented with bloody diarrhea, tenesmus, and high fever. Endoscopic findings were compatible with an acute attack of ulcerative colitis, which proved to be resistant to systemic corticosteroid treatment. In the presence of an acute abdomen with ascites and double-contoured colonic wall, hemicolectomy was performed. Postoperatively, high temperature, hyponatremia, and elevated liver enzyme levels persisted. Pleural effusions developed. Antibodies to Legionella pneumophila serogroup 3 were detected in the serum. Erythromycin therapy induced rapid improvement. In a massive submucosal edema of the affected colon, L. pneumophila of the same serogroup was demonstrated by direct immunofluorescence staining.