The occurrence of Langerhans cell histiocytosis (LCH) and another malignancy in the same patient is infrequent but has been recognized. The genetic changes that could be responsible for LCH and/or concomitant leukemia development are obscure. To the best of our knowledge, this is the first description of constitutional maternally derived inv (9) (p12;q13) in an LCH patient, and also of the development of common ALL Ph after LCH diagnosis and therapy. The potential significance of these findings [inv (9)LCHALL Ph] and their mutual relationship are unknown. Therefore, cooperative studies of large numbers of patients are needed to identify the common risk factors, if any.