Twenty-one drug-using youths (10 males and 11 females) were recruited from residential and outpatient drug treatment centers; and 21 drug abstainers were recruited from the social networks of the drug-using youths and from community organizations that were strongly opposed to drug use. All youths were assessed on several psychosocial variables by using a structured interview schedule, a set of questionnaires, and a situation role-play test of social skills. First, concurrent validity of the POSIT was tested by comparing POSIT domain scores between these two groups of youth. Based on the analysis, all 10 POSIT domain scores for drug-using youth were greater than those for the abstaining youths; and 9 of the 10 were significantly greater. Second, construct validity was evaluated by comparing POSIT domain scores with scores from other instruments. Pearson's bivariate correlations were used to test the convergent validity of the POSIT domains with the same or similar constructs. Strong support for the convergent validity of three domains (Substance Use, Mental Health, Aggressive Behavior/Delinquency), some support for four domains (Physical Health, Family Relations, Educational Status, and Social Skills), and minimal support for three domains (Peer Relations, Vocational Status, and Leisure and Recreation) was found. The analysis also showed discriminant validity of the POSIT domains by generating nonsignificant correlations with nonrelevant and dissimilar variables. Overall, the POSIT exhibits excellent clinical utility as well as concurrent and construct validities; further, it is an effective screening instrument with at-risk youth. 13 tables and 44 references