Subtrochanteric osteotomy has been reported to interrupt the destructive phase of coxa plana and speed up the rate of healing. The aim of this investigation was to test the validity of these statements. The material comprised twenty-five patients treated by osteotomy and thirty-three patients treated by support in a Thomas's splint. For every patient the extent of the radiological changes was estimated and plotted against time as a percentage of the normal epiphysial volume. The graph so obtained represents the rate of destruction and re-ossification, as well as the maximum involvement of the epiphysis. The curves for each group of patients were superimposed in a graph with all the minima coincident in time. The two patterns of curves are closely similar. It was confirmed statistically that the rates of destruction and re-ossification did not differ between the two groups. The results afford no evidence that osteotomy interrupts the disease or speeds up healing.