Background. Legg-Calvé-Perthes disease, due to its frequency and impact on hip function, is an important orthopedic issue. The goal of this research was to assess long-term outcome after conservative treatment in Perthes' disease. Material and methods. We studied 123 patients (135 hips), 105 males and 18 females, average follow-up 19.5 years (range 10-34), average age at onset 6.4 years. Conservative treatment involved non-weight bearing, abduction casts, orthoses and rehabilitation. The average treatment duration was 2.9 years. The range of hip motion, pelvic stability, limb length, pain, limping and physical activity were estimated. The patients were classified according to Catterall's criteria and age of onset. In x-rays the risk factors, Wiberg angle, and acetabulum-head index were measured. Outcome was assessed according to Mose and Stulberg, and arthritic changes were noted. Results. At follow-up there was full physical activity in 50 cases (40.7%). Limping was observed in 38 patients (30%), limb shortening in 27 (22%). The range of motion was limited in 37 hips (27%), and 13 hips were unstable (9.6%). There was a good Mose outcome in 36 hips (26.7%), fair in 70 (51.8%), and poor in 29 (21.5%). By Stulberg's classification, 35 hips were in class I (25.9%), 49 in class II (36.3%), 31 in class III (23%), 11 in class IV (8.1%), and 9 in class V (6.7%). Arthrotic changes were noted in 51 hips (37.8%). Conclusions. Twenty years after onset only 25% of the evaluated hips were normal. This suggests low effectiveness of treatment.