Subjects were recruited from two public adolescent drug misuser service centers in St. Paul, Minnesota, and San Jose, California. One-year test-retest reliability of PEI Problem Severity Scales was measured in a sample of 37 drug-misusing adolescents who did not receive treatment during the 1-year test- retest interval. The five PEI Problem Severity Scales demonstrated satisfactory 1-year temporal stability coefficients, ranging from 0.86 to 0.89. The utility of the PEI as a measure of change was demonstrated by using 1-year test-retest reliability data to measure both statistically significant and clinically significant change between admission and 1-year followup in a sample of 45 drug-misusing adolescents who received treatment. The PEI was useful in identifying which treatment clients exhibited statistically significant improvement, statistically significant deterioration, and no reliable change. An appendix contains sample items from the five PEI Problem Severity Scales. 18 references, 2 tables, and 2 figures