Substance abuse treatment programs are being pressed to measure and make clinical decisions more efficiently about an increasing array of problems. This computerized adaptive testing (CAT) simulation examined the relative efficiency, precision and construct validity of different starting and stopping rules used to shorten the Global Appraisal of Individual Needs’ (GAIN) Substance Problem Scale (SPS) and facilitate diagnosis based on it. Data came from 1,048 adolescents and adults referred to substance abuse treatment centers in 5 sites. CAT performance was evaluated using: (1) average standard errors, (2) average number of items, (3) bias in personmeasures, (4) root mean squared error of person measures, (5) Cohen’s kappa to evaluate CAT classification compared to clinical classification, (6) correlation between CAT and full-scale measures, and (7) construct validity of CAT classification vs. clinical classification using correlations with five theoretically associated instruments. Results supported both CAT efficiency and validity.