The major clinical investigation employing fast neutrons in the treatment of invasive bladder cancer are reviewed. Although data suggest that preoperative radiation schedules employing neutrons may result in a greater degree of pathologic downstaging than conventional precystectomy photon regimens, this has not led to an improved survival rate for neutron-treated patients over photon-treated patients. Randomized clinical trials comparing primary neutron irradiation and primary photon irradiation do not disclose an advantage for neutrons over photons as measured by survival rate or freedom from local tumor recurrence. The late complications in normal pelvic tissues following neutron irradiation with low-energy beams exceed those experienced after photon irradiation and have led to an unexpectedly high rate of treatment-related morbidity and mortality. A partial explanation for the toxicity may be attributed to the use of neutron beams with poor depth dose characteristics for the treatment of what is a deep-seated malignancy. An additional explanation is the documented lack of a differential in radioresponsiveness to neutrons between the bladder primary tumor and adjacent normal pelvic tissues.