Fifty patients up to 15 years of age with simple bone cysts were treated at the Department of Pediatric Surgery at the Children's Hospital Zagreb. There were 35 boys and 15 girls with 26 humeral cysts, 15 femoral cysts, 8 tibial cysts, and 1 radial cyst. There was one case of multilocularity of a simple bone cyst (in the humerus and femur). Among those cysts, 19 were multicameral (in the femur and humerus). The size of cysts was determined with regard to the length of the affected bone. Cysts involving up to one tenth of the bone length were defined as small, those up to one fifth as medium, and those exceeding one fifth of the bone length as large. Small cysts were followed up, medium ones were treated surgically in case they did not heal after pathologic fracture, and large cysts were treated surgically without delay. The surgery included opening the cyst wall, curetting the wall epithelium, rinsing with peroxide, opening the medullar canal, and filling the cavity with lyophilized bone. If necessary, a thin "old" Küntscher nail was inserted to achieve stability of fragments and to keep the medullar canal free. The medullar canal was opened in 38 surgically treated patients, and there were 36 good results. In the patients who had the medullar canal opened during surgery, there were fewer recurrences. Healing time was shorter than reported in the literature, even more so if the thin Küntscher nail was used.