Forty-eight images using magnetic resonance imaging (MRI) in 16 hips with Perthes' disease were evaluated over a mean period of two years. MRI depicted exactly the infarcted zone in the femoral head before typical radiological changes were evident. Early determination of the extent of the infarcted bone on MRI benefits those patients who require treatment. Follow-up MRI scans at six-monthly intervals, reflected the chronological stages of the repair process in each group classified according to Catterall.