We describe a patient with growth failure and multiple congenital anomalies characteristic of Fanconi anaemia, but without the classical feature of progressive bone marrow hypoplasia. Following treatment with growth hormone for a period of 8 years, he presented with myelodysplastic syndrome and a karyotypically abnormal clone in the bone marrow (47,XY,+8). The diagnosis of Fanconi anaemia was supported by the induction of abnormally high levels of characteristic chromosome aberrations in peripheral lymphocytes following exposure in vitro to the bifunctional alkylating agent mitomycin C. Immune function studies also identified a selective IgA deficiency. The relative importance of interacting constitutional and exogenous factors involved in the development of preleukaemia in this patient is discussed.