Three reported cases of Creutzfeldt-Jakob disease (CJD) in young adults who had received human growth hormone (hGH) raised concerns that pituitary-derived GH had been contaminated. Subsequently reported cases have confirmed this suspicion. The US Public Health Service is conducting an investigation to determine the extent of the problem of CJD in recipients of National Hormone Pituitary Program (NHPP) GH. In addition, other possible adverse effects of GH use including leukemia are being investigated. The design, conduct and current status of the study are the subject of this report. Interview data are now available on 5,240 of the 6,284 subjects treated by the NHPP for growth problems. Analysis is underway.