This paper discusses the rationale of fast neutron therapy and presents some early results obtained at the Medical Research Council's Cyclotron Unit at Hammersmith Hospital, London. The importance of the oxygen enhancement ratio for fast neutrons is considered, together with the significance of the small proportion of anoxic cells present in most tumours. The significant features of the preliminary results obtained in patients with malignant disease are presented and the characteristics of fast neutron beams for radiotherapy are discussed.