SummaryForty-three patients with unilateral severe painful patellae, fulfilling the diagnostic criteria for 'chondromalacia patellae' laid down by Robinson and Darracott (1970), took part in a double-blind trial, 23 receiving an anabolic ester–nandrolone phenyl-propionate (‘Durabolin’). All patients followed a progressive quadriceps exercise programme. Twenty of the 23 patients given the anabolic ester were clinically improved. Changes in bone density in skyline knee radiographs were measured with a simple apparatus described.In 17 of the 23 patients given the anabolic ester, the bone density measurements improved. The 20 patients given the placebo showed an almost complete lack of response to the exercise programme alone. The possible aetiological factors involved in these results are discussed.