The authors discuss the treatment of 6 cases of extensive bone cysts in young patients (10-18 years of age) by elastic endomedullary nailing, all of which were successful, with complete remodelling of the bone. They advance the hypothesis that this was probably due to dispersion of the fluid by drainage along the nails, followed by natural remodelling in these young patients. The technique is illustrated. The advantages claimed are its simplicity of execution, and the reliability of the results.