This study compared alcohol consumption data collected through daily interviews on a smartphone with data collected via the Timeline Follow-Back (TLFB) during a 6-week pilot study of a smartphone-based alcohol intervention system. The focus of the study was to assess for discrepancies between the two measurement methodologies on commonly utilized alcohol outcome variables. Twenty-five participants between 22 and 45 years of age and were drinking heavily at study incipience completed a 6-week alcohol intervention delivered by a smartphone application that monitored drinking through a daily interview. Participants also completed a TLFB of their alcohol consumption during the intervention period. Difference scores were calculated by subtracting the average weekly outcome variables derived from the smartphone daily interview from the average weekly outcome variables derived from the TLFB which yielded six discrepancy scores for each of the 25 participants and resulted in 150 observations. Heirarchical linear modeling indicated that the TLFB and smartphone daily interview methods did not produce significant discrepancies over the 6-week interval when examining percent of heavy drinking days and percent of days abstinent. However, discrepancies on drinks per drinking day increased substantially over time, suggesting that it is susceptible to recall bias when assessed by the TLFB. Results also indicated that participants under-reported their drinking on the TLFB compared to the daily smartphone-based assessment. Results indicate that outside of assessing for extreme drinking (binge or no drinking), the TLFB should be used cautiously and that smartphone apps represent a promising method for accurately assessing alcohol consumption over time.