The results of a randomized trial of d(15)+Be neutrons compared with 4 or 6 MV photons for the treatment of transitional cell carcinoma of the bladder. Between December 1978 and December 1981, 113 patients were accrued, 53 allocated to be treated by neutrons and 60 by photons. Complete local tumor regression was observed in 64% of patients treated by neutrons and 62% treated by photons. Recurrent cancer was subsequently confirmed in 31% of patients, similar in both treatment groups. There was no significant difference in the control rates by T stage between the two treatment groups. Late morbidity was significantly worse in patients treated by neutrons. Following neutron therapy, 78% of patients had serious late morbidity in at least one tissue compared with 38% in the group treated by photons. Survival was significantly better in the photon treated group 45.3% (+/- 11%) at 5 years compared with 12% (+/- 6%) after neutron therapy.