Hardcore drug use is defined as the use of heroin, powder cocaine, or crack cocaine on 8 or more days during at least one of the preceding 2 months. The approach described here involves interviewing admitted hardcore users at locations where they are most likely to be found in substantial numbers, e.g., jails, drug treatment programs, and homeless shelters. The users are questioned about the characteristics of their drug use careers and the frequency with which they made contact with various kinds of institutions during some preceding period of time. The total number of contacts divided by the estimated rate of contact yields an estimate of the size of the hardcore drug using population. This approach produced estimates of about 333,000 based upon arrests, 318,000 based upon treatment admissions, and 53,000 based upon shelter stays. While these estimates should be treated with some caution, the shortcomings of the data do not challenge the basic finding of the study; the approach is feasible and appears to address the known limitations of conventional survey research. Tables, notes, figures, bibliography