Since 1988 the number of growth hormone (GH)-treated patients has markedly increased worldwide. To date, leukemia has been observed in 31 patients during or following GH therapy and related malignancies in 2 further patients. Leukemia occurred in 10 patients in Japan, 10 in the USA, and 10 in Europe, and in 1 patient in Canada. In 29 patients GH therapy had been started in 1975 or later. The onset of leukemia was 1984 or later in 28 patients with a mean time between the start of GH therapy and leukemia onset of 5.0 (0.2-18.8) years. Patients had received both pituitary and recombinant GH in moderate doses. In 15 patients definite additional leukemia risk was evident: Fanconi anemia in 2, myelodysplastic syndrome in 1, Bloom's syndrome in 1, radiation for brain tumor (+chemotherapy) in 9, chemotherapy in 2. The leukemic patients without a strong additional risk do not represent a definitely higher leukemia incidence worldwide, except for Japan where the occurrence is higher than expected.