The Harper Hospital and Wayne State University fast neutron therapy facility is the only one in the world to use a compact superconducting cyclotron and multirod collimator. Neutrons are produced by the interaction of the 48.5-MeV deuteron beam with a thick internal beryllium target and the compact accelerator is gantry mounted to allow full 360 degrees rotation of the neutron beam about the therapy couch. The deuteron beam strikes the beryllium target at a glancing angle. A flattening filter is used to flatten the asymmetric neutron beam which results from this geometry. Details of the flattening filter design and construction are discussed. The physical characteristics of the resulting neutron therapy beam were measured. The central axis depth-dose values are approximately equivalent to those of a 4-MV photon beam. The dose buildup curve reaches its maximum value at a depth of 9 mm in a water phantom and the surface dose is approximately 42%. The beam penumbra produced by the multirod collimator has been measured in terms of the distance between the 20% and 80% isodose lines. The penumbra width for a 10 x 10-cm2 field at a depth of 10 cm in a water phantom is 1.65 +/- 0.1 cm, and is comparable to that achieved with other high-energy neutron beams. The long-term stability of the dose monitoring system has been measured and found to be satisfactory. The physical characteristics of the neutron beam are comparable with those of other modern fast neutron therapy facilities.