Hair assay revealed that 50 of the 88 subjects (56.8 percent) had used cocaine; concentration levels were generally moderate to high. In contrast, urinalysis results identified only seven subjects (8 percent) as having recently used cocaine. Cross-tabulations of urinalysis and sectioned hair assay results show that the two detection methods were in greatest concordance for subjects who were heavy users of cocaine and who used cocaine in the last 30 days (as determined by hair assay). Even for these subjects, however, concordance was modest. Self-reports of drug use yielded severe underestimates of the prevalence of cocaine use in this population. In effect, the findings show that many who use cocaine often or in large amounts (as occur during drug "binges") apparently find themselves often without the means, opportunity, or desire to use cocaine and remain thus indisposed to cocaine for at least 2 to 3 days at a time. It is at this stage in their career that drug users, particularly juvenile drug users, may be most receptive to drug intervention and education. Without it, many of these apparently intermittent cocaine users will become daily cocaine users, willing to undertake whatever measures are necessary, including further criminal activity, to obtain and consume cocaine at an increasing rate. 9 tables and 16 references