Following an initial report from Japan in 1987, 15 growth hormone (GH)-deficient patients developed leukaemia during or following GH treatment. Nearly all available pituitary and biosynthetic growth hormones have been used. In 14 of these 15 patients GH treatment was initiated in 1975 or later with doses between 4.5 and 18IU/m2 per week. The therapy period was between 0.17 and 8.0 years. Leukaemia occurred 0.2-11 years after the start of GH treatment. GH affects normally and abnormally growing blood cells in vitro and in animal experiments, but the clinical data in humans do not indicate GH induction of tumour growth. Seven out of the 14 patients under discussion had an additional increased leukaemia risk. Two other patients had been treated only for a very short time. Though no clear evidence of a strikingly augmented leukaemia incidence in GH-treated patients is found worldwide, the available data call for increased attention.