Many substance users deny their substance use to avoid negative consequences, thus diluting the accuracy of assessment. To address this issue, indirect items are often included on substance use measures to identify those who deny their use. The purpose of this study was to examine the effect of complete denial and partial denial on substance use measures. Partial denial, also termed denial of effects, is the denial of substance use interfering in multiple domains of a person's functioning. The study used a mixed within- and between-subjects design with participants from a dual diagnosis inpatient unit. Each participant completed the study under two different conditions which include an honest condition and an experimental condition (either complete denial or partial denial). Results show that partial denial is distinctly different from complete denial across three self-report substance use measures. Importantly, substance users engaging in these denial conditions were often undetected by these measures.