The effectiveness of spinal manipulation carried out by a non-medical qualified osteopath was compared with that of short-wave diathermy (SWD) and a placebo (detuned SWD) in 109 patients with low back pain. More than half the subjects in each of the 3 treatment groups benefited immediately from therapy. Significant improvements were observed in the 3 groups at the end of 2 weeks' treatment, and these were still apparent at 12 weeks. The outcome of treatment was unrelated to the initial severity or duration of pain or to the trend of pain towards deterioration or improvement. It is, therefore, unlikely that the results simply reflect the natural history of low back pain. Benefits obtained with osteopathy and SWD in this study may have been achieved through a placebo effect.