Eleven patients between the ages of 6 and 18 years who had been treated for acute leukemia were investigated for growth and growth hormone (GH) secretion. All had undergone bone marrow transplantation (BMT) between 0.7 and 5.1 (median 2.0) years previously. Preparation of patients for BMT had included high-dose cyclophosphamide and total body irradiation. In the eight patients at risk of growth failure, the relative height decreased 0.5-2.5 SD units (median 1.0) during the follow-up period. Eight patients secreted subnormal amounts of GH as studied by measuring spontaneous pulsatile GH secretion overnight. The maximal nocturnal GH peak varied between 3.3 and 28.3 micrograms/l (median 9.3). The mean nocturnal GH concentration varied from 1.2 to 8.3 micrograms/l (median 2.3) and depended on the length of the follow-up period. We conclude that deficient GH secretion is one reason for poor growth after BMT. A good growth response to GH substitution would support the role of GH deficiency in the observed growth retardation after BMT.