To assess the value of the current NIDA cutoff concentrations for screening assays that detect urinary cocaine metabolites and phencyclidine (PCP), we collected data on concentrations of these drugs in newborns and patients admitted to the Los Angeles County + University of Southern California Medical Center from July 1, 1991, to December 31, 1991. Less than 2% of the patients were positive for PCP. However, 16.5% of the newborns and 25.1% of the remaining age groups tested positive for cocaine metabolites. Among specimens that tested negative with the screening assays, approximately 3% (182 specimens, with 15 from newborns) clearly contained detectable amounts (between 50 and less than 300 ng/mL) of benzoylecgonine by GC/MS, while less than 0.6% had detectable amounts (10-25 ng/mL) of PCP. The mothers of 7 of the 15 newborns also had detectable benzoylecgonine at various concentrations. This indicates that a lower screening cutoff concentration may be desirable for cocaine metabolites in hospitalized patients. Among those patients positive for cocaine metabolites or PCP, most were males between 20 and 39 years old. The urine drug concentrations in this population were 4-300 times greater than the cutoff concentrations for the screening assays.