Creutzfeldt-Jakob disease in pituitary growth hormone recipients in the United States. To assess the magnitude of Creutzfeldt-Jakob disease (CJD) occurrence among recipients of pituitary-derived human growth hormone (HGH), we conducted an epidemiologic follow-up of 6284 recipients of HGH distributed through the National Hormone and Pituitary Program. Seven neuropathologically confirmed cases of CJD have occurred in this population to date: six patients with clinical CJD presented with ataxia and imbalance, rather than with altered mentation, which is the most common initial manifestation in sporadic CJD, and one patient died in the preclinical incubation state of the disease. All seven cases occurred among the nearly 700 HGH recipients who started therapy before 1970. Since only 10% of the cohort has been followed up for the 15-year average incubation interval from midpoint of HGH treatment to onset of symptoms, the great majority of potentially exposed patients have not yet attained the requisite incubation period for expression of CJD. The median duration of HGH therapy of 100 months in the CJD cases was significantly longer than 41 months for all patients starting treatment before 1970; thus, the duration of pituitary HGH therapy is a major risk factor for CJD. 