This prospective study was performed to determine the true incidence of deep vein thrombosis of the lower limb in children who had undergone halo-femoral traction to correct scoliosis before operation. Bilateral ascending phlebography of the lower limbs was performed on 54 children two days before spinal fusion and Harrington rod instrumentation. Two patients developed clinical evidence of thrombosis whilst on traction. This diagnosis was confirmed by phlebography. The other 52 children had no clinical evidence of thrombosis and their venograms were normal. The incidence of thrombosis was 3.7 per cent and clinically silent thrombosis did not occur.