BACKGROUND: The one-month Time Line Follow Back calendar (TLFB) and the Alcohol Use Disorders Identification Test (AUDIT) are used to collect self-reported alcohol intake data. We compared these instruments with the alcohol biomarker phosphatidylethanol (PEth) among young-people in northern Tanzania. METHODS: AUDIT and TLFB were applied in a cross-sectional study of 202 young people (18-24 years), who reported using alcohol during the past year (103 male casual labourers; 99 college students). We assayed whole blood for PEth 16:0/18:1, using liquid chromatography-tandem mass spectrometry. RESULTS: For both self-report methods, alcohol consumption was high, particularly among men (e.g. a median of 54 drinks per month in labourers), and about half of male students (48%) reported hazardous or harmful levels of drinking (AUDIT ≥8). Almost half (49%) of participants were PEth-positive (median concentration 0.03μmol/L). There were significant positive correlations between reported total alcohol intake and PEth concentration in males (Spearman's correlation rs=0.65 in college students and rs=0.57 in casual labourers; p<0.001). Self-reported use in the past month was a sensitive marker of having a positive PEth result (≥0.01μmol/L) with 89% of those with a PEth positive result reporting alcohol use, and this was similar in all groups. The proportion of those with AUDIT scores ≥8 and AUDIT-C scores ≥6 among those with a high cut-off positive PEth result (≥0.30μmol/L) ranged between 94 and 100%. CONCLUSION: TLFB and AUDIT are sensitive measures to detect heavy alcohol use among young-people in northern Tanzania. They can be used to identify young people who may benefit from alcohol-focused interventions.