In the decade 1972-1982, in vivo nuclear medicine procedures in the United States increased from 3.3 million to about 7.5 million per annum. This growth has been the result of a markedly increased frequency in some types of examinations; particularly bone, liver, lung, and cardiovascular imaging. The only type of imaging in which a decrease in frequency has been observed has been in radionuclide brain imaging. Examination of these trends illustrates the difficulty in forecasting, even over time periods as short as 5 years. Competing tests have largely replaced radionuclide imaging in some areas; although in other areas, such as cardiac nuclear medicine, competing technologies appear to have been additive in terms of the frequency of examinations. Comparison with recent data from other countries indicates that the frequency of nuclear medicine procedures in the United States is probably the highest in the world.