Heroin activity in New York City is traced from 1970 using a variety of indicators. A steady decline in the number of new "intensive" users (those who required treatment) is noted, beginning around 1970. Various possible explanations of this decline are examined. Reductions in the supply of heroin or in the demand for heroin do not appear to be causes of the decline in new users, as these reductions occurred after the decline in new users had already begun. Demographic changes--the passing of the baby boom cohort through the period of highest risk of beginning heroin use--also does not appear to be a viable explanation, as trends in use of other drugs do not show a decline. Changes in attitudes toward heroin as a specific drug are left as the most plausible explanation of the decline in the number of new intensive heroin users.