We have compared whole body skeletal scintigraphy with X-ray examination in 15 children suffering from histiocytosis X, 14 of whom also showed involvement of the skeleton. In 9 of 20 cases, relapses, included, involvement of the skeleton was found both in scintigraphy and X-ray examination. In 7 cases only X-ray examinations have revealed lessons, which could not be proved by scintigraphy. Vice versa, in 4 cases scintigraphy was positive alone. These results confirm the necessity to use both methods for an exact documentation of the extent of the disease before undertaking any therapy. Dispensing with one of the two methods may cause incorrect therapeutic decisions and increase the risk of relapses and late complications.