Until the middle 1960s, almost all reports in the literature relating to the safety of radio-pharmaceuticals referred only to radiation hazards. Accounts of specific adverse effects were also of this nature, usually an accidental over-exposure to radiation. A scattered literature later began to appear recording adverse reactions to the vehicle carrying the source of radiation and similar reports were from time to time mentioned among colleagues.During approximately the last decade there has been an explosive increase in the clinical usefulness of imaging using radioactive tracers. In part, this has been a result of improved instrumentation but a significant factor has been the introduction of a variety of vehicles for the radioactive tracer chosen. During roughly the same period, methods have been developed to make short-lived nuclides available at the site of use by elution from a column containing the longer-lived parent. This gave new impetus to the search for chemicals having desirable organspecific qualities which could be quickly and easily labelled with such nuclides. Thus there arose the current, still expanding, situation of a wide range of materials of various chemical and physical forms being in routine clinical use.The Nuclear Medicine Committee of the British Institute of Radiology decided, in 1972, to recommend to the Council of that Institute that it undertake informally to circulate a questionnaire to known users of radio-pharmaceuticals. This recommendation was accepted and the questionnaire was sent to 73 individuals.