Between 1983 and 1996, nine children were treated with extensible endoprosthetic replacements of the proximal femur after resection of primary bone tumors. Four patients died of their disease. The remaining five patients were observed for an average follow-up period of 7.6 years. In these five patients, the authors performed an average of 10.2 operations per patient, including five lengthenings and a mean total extension of 69.7 mm per patient. Acetabular loosening and hip dislocations were the most frequent complications. Only two patients have not had a revision or a major complication. Despite this, four children are alive with a functioning lower limb and a mean Musculoskeletal Tumor Society functional score of 77.6%. Extensible endoprosthesis of the proximal femur is a viable alternative to hip disarticulation and in selected children offers an opportunity for near-normal development of the lower limb by allowing equalization of limb length and the ability to walk without the use of mobility aids.