Since 1960 more than 30,000 children have been treated with growth hormone (GH) in the USA. Ten cases of Creutzfeldt-Jakob disease have been associated with the use of GH purified from pituitaries in the USA, and more cases may appear in the future. Ten cases of leukemia or preleukemia have been reported in patients undergoing GH treatment in the US. Eight of these patients had previously diagnosed tumors of the central nervous system. As the indications for GH treatment broaden, and the dosages of GH increase, more unfavorable clinical events linked directly to the biological actions of GH can be expected to occur. Continued surveillance for clinically important GH-associated events is important.