BACKGROUND: Growth hormone raises serum concentrations of insulin-like growth factor IGF-I, which is mitogenic and antiapoptotic. There is evidence that raised endogenous levels of growth hormone and IGF-I might be associated with increased risk of certain solid cancers, but there have been no data on long-term risks of solid cancers after growth hormone treatment. METHODS: We did a cohort study to investigate cancer incidence and mortality in 1848 patients in the UK who were treated during childhood and early adulthood with human pituitary growth hormone during the period from 1959 to 1985. Patients were followed up for cancer incidence to December, 1995 and for mortality to December, 2000. Risk of cancer in the cohort was compared with that in the general population, controlling for age, sex, and calendar period. FINDINGS: Patients treated with human pituitary growth hormone had significantly raised risks of mortality from cancer overall (standardised mortality ratio 2.8, 95% CI 1.3-5.1; ten cases), colorectal cancer (10.8, 1.3-38.8; two cases), and Hodgkin's disease (11.4, 1.4-41.3; two cases). Incidence of colorectal cancer was also greatly raised (7.9, 1.0-28.7). After exclusion of patients whose original diagnosis rendered them at high risk of cancer, the significance and size of the risks of colorectal cancer incidence and mortality, and of Hodgkin's disease mortality were increased. INTERPRETATION: Although based on small numbers, the risk of colorectal cancer is of some concern and further investigation in other cohorts is needed. We have no evidence as to whether growth hormone in modern dosage regimens is associated with an increased risk of colorectal cancer.