The sample was interviewed and their criminal history was traced in detail over an 11 year risk period during which they were 'on the street.' It was found that these 243 heroin addicts had committed more than 473,000 crimes. As measured by crime-days, the average addict committed over 178 offenses per year and almost 2,000 offenses during his post-onset lifetime. Although the predominant offense committed was theft (as with most populations of criminals), these addicts were also involved in a wide range of other crimes; drug sales, robbery, forgery, pimping, assault, and murder. The implications of these findings are discussed with respect to the impact of arrest and incarceration upon the lifetime criminality of these addicts. These research results provide a means for estimating the extent of criminality among heroin addicts throughout the United States. Our calculations indicate that the 450,000 heroin addicts in the United States commit more than 50,000,000 crimes per year and that their lifetime criminality exceeds 819,000,000 offenses. (Author abstract)