The Timeline Followback (TLFB; L. C. Sobell & M. B. Sobell, 1996) interview, which uses a calendar method developed to evaluate daily patterns and frequency of drinking behavior over a specified time period, has well-established reliability and validity for assessing alcohol consumption. Although several investigators have used the TLFB to evaluate drug-using behavior, few studies have examined the psychometric properties of the interview for this purpose. The authors conducted TLFB interviews with a sample of adult drug-abusing patients seeking treatment for substance abuse (n = 113) at baseline, posttreatment, and quarterly thereafter for 12 months. It was found that the patients' reports about their drug consumption using this method generally had high (a) retest reliability, (b) convergent and discriminant validity with other measures, (c) agreement with collateral informants' reports of patients' substance use, and (d) agreement with results from patients' urine assays.