AbstractNeutrograms of patients are being taken during the course of radiotherapy with the fast neutron beam from the MRC cyclotron. A combination of industrial X ray film with radiographic salt screens is used so that the cassette can be left in position during most (or all) of a treatment. Fogging by scattered neutrons and γ rays is reduced by use of a patient-film separation of 20 cm.Bone is practically invisible on neutrograms which show only the contrast between tissue and air spaces. It is doubtful, however, whether this fact would enable any anatomical information to be visualized which could not be shown with X rays using a suitable kV and projection. Fundamental problems in the diagnostic use of neutron radiography are the insensitive imaging techniques, leading to high dosage to the patient, and increased mottle in neutron pictures.