The reliability and validity of self-report data regarding substance abuse has often been questioned. To determine how best to enhance the veracity of self-report, three factors which might affect self-report veracity were examined: alcohol status at time of interview; level of cognitive functioning; and method of self-report data collection. Subjects were 234 admissions to an inpatient substance abuse treatment unit. Self-report data were collected via both personal interview on the day of admission and and questionnaire within the first week of stay. Self-reports concerned use of alcohol, cocaine, and marijuana in the days preceding admission. Test-retest reliability for the questionnaire data produced reliability coefficients of 0.88, 0.91, and 0.88, for alcohol, cocaine, and marijuana, respectively. Variation in inter-test interval had virtually no effect upon reliability coefficients. Interview data were compared to toxicologic analyses of blood and urine samples collected on admission. Overall, this comparison showed self-reports to be valid, with a 97% agreement between verbal report and laboratory data for alcohol, 93% for cocaine, and 84% for marijuana. The comparison of interview data with questionnaire responses also showed self-reports to be valid: 90% agreement for alcohol, 93% for cocaine, and 81% for marijuana. Level of cognitive function did not influence the validity of self-reports for any of the three substances. Recent consumption of alcohol also had no statistically significant effect on the validity of self-reported marijuana use, regardless of the operational form of validity tested. However, BAC-negative subjects produced a significantly greater validity coefficient for self-reported cocaine use (kappa = 0.87) than did BAC-positive patients (kappa = 0.43), when interview data were compared with toxicologic measures. A similar finding was not uncovered when interview and questionnaire data were compared. An interaction between admission alcohol status and cognitive function was uncovered for cocaine self-reports when interview data was compared with toxicologic measures. The rate of agreement for alcohol-negative subjects is quite high for both cognitively impaired and unimpaired subjects (M = 93% and M = 94%, respectively) as well as for alcohol-positive, cognitively unimpaired subjects (M = 94%), but not for alcohol-positive, cognitively impaired subjects (M = 67%). Results are discussed in terms of threats to the validity of self-report and strategies for the optimization of response accuracy.