Probably congenital histiocytosis X with unusual radiographic findings in a 7-week-old infant.

A 7-week-old infant who probably had congenital histiocytosis X of the Letterer-Siwe type, and who showed mesomelic involvement of the long bones, is reported. Although skeletal changes are common in histiocytosis of the Letterer-Siwe type, very early appearance and localisation of these changes is unusual.