The report documents the data collection methodology and procedures of a follow-up survey conducted of persons under 16 years old who received diagnostic Iodine 131 for evaluation of thyroid function at nine clinical centers prior to December 31, 1960. The intent of this data collection effort is to estimate the dose response curve for the development of thyroid neoplasms in young adults who received low diagnostic doses of Iodine 131 as children. Of 2,287 potential study subjects identified, some medical records were reviewed and abstracted for 1,999 or some 87 percent. With study activities incomplete, of 186 private physicians and other referral sources contacted for supplemental data, only three or less than two percent declined to participate; of 1,362 patients who were determined eligible and entered the survey phase, some final resolution (completed questionnaire, death certificate, or refusal) was obtained for 1,065 or some 78 percent. The statistical analysis of these data will be performed under a related project sponsored by the U.S. Food and Drug Administration.