The purpose of this trial was to evaluate the response of patients with osteoarthrosis of the knee to deep X-ray therapy by comparison with the results obtained with standard physiotherapy. Patients selected for inclusion were aged 60 years or more and were randomly allocated blind to one of two treatment groups. Those given radiotherapy received 800 rads over four weeks. Physiotherapy comprised short-wave diathermy combined with standardized exercises, the course also lasting four weeks. Assessments were made before and after treatment and then 10 and 24 weeks after the start of treatment. Note was made of subjective alleviation of pain and stiffness; changes in analgesic consumption; changes in range of knee flexion and an estimate of the volume of synovial effusion. Functional grading was assessed according to the patients' ability to stand from the sitting position. Twenty-nine subjects underwent radiotherapy and 17 received physiotherapy. The results revealed no significant differences between the treatment groups.