Photodynamic therapy involves light-induced destruction of tumors containing a photosensitizer such as hematoporphyrin derivative. We conducted a collaborative study to evaluate the efficacy of this form of therapy in treating superficial transitional-cell carcinoma of the bladder. Thirty-seven patients were evaluated and 20 were selected for treatment. A total of 50 papillary tumors and 3 areas of carcinoma in situ were treated. All except two tumors were smaller than 2.5 cm. Assessments for treatment response and toxicity were carried out three months after treatment. The initial diagnosis of one patient was revised after the biopsy material was reviewed, and this patient was not included in the analysis. Complete eradication of all tumors was observed in 9 of 19 patients (47 percent), including those with carcinoma in situ. In the remaining 10 of these 19 patients, 13 tumors could not be eradicated (the overall eradication rate was 37 of 50 tumors [74 percent]), but 9 of the 10 patients had a reduction in tumor size, number, or both of 50 percent or more. We conclude that photodynamic therapy is useful in the treatment of superficial transitional-cell carcinoma of the bladder, but controlled trials will be required to define its place in the treatment of cancer.