Thirty-two patients (thirty-seven hips) who had a so-called acute-on-chronic or chronic slipped capital femoral epiphysis were treated with traction for relief of symptoms and then with immobilization in a spica cast for eight to sixteen weeks. The disappearance on radiographs of a metaphyseal juxtaphyseal radiolucency, rather than closure of the physis, was used as the criterion for removing the cast. In one (3 per cent) of the thirty-seven hips, the slip progressed; possibly this could have been prevented by keeping the cast on for a longer period of time. Narrowing of the cartilage space was seen after treatment in a cast in seven (19 per cent) of the thirty-seven hips. In five of these seven hips, this was true chondrolysis; in one, the diagnosis of chondrolysis had been apparent before treatment. Avascular necrosis did not develop as a result of treatment in any patient. Treatment in a spica cast should be considered as an alternative for patients who have an acute-on-chronic or chronic slipped capital femoral epiphysis.