Clients just entering treatment for nonalcohol primary drug use were asked to report on their frequency of drug use at two times 2 days apart. Test-retest correlations for the lifetime use of drugs were fair to excellent, and those for drug use in the last 90 days, although variable, were generally very good. For lifetime use, correlations were highest for opiates and stimulants and lowest for inhalants, whereas for the recent period correlations were highest for opiates and lowest for stimulants. Correlations were uniformly high for measures of general life functioning. Initial validity assessments with urine drug screen results as criterion were good, with no false negative errors in four of six drug use categories. Overall Form 90D appears to be a reasonably reliable and valid interview instrument for measuring drug use occurrence and frequency for both lifetime and recent use. Care is warranted in assessing classes of drugs that are used less frequently.