The distribution of hematoporphyrin derivative has been studied in patients undergoing transurethral resections and total cystectomy for carcinoma of the bladder. Although hematoporphyrin derivative fluorescence was not confined to malignant tissues with the dose used there was preferential localization in malignant and pre-malignant epithelium. Also high intensity illumination of an area of hematoporphyrin derivative sensitized carcinoma caused tumor destruction. Therefore, it is suggested that hematoporphyrin derivative could be used as an aid to the diagnosis and treatment of carcinoma of the bladder.