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Phosphatidylethanol (PEth) is a recently introduced biomarker with high specificity, high sensitivity, and response correlating with alcohol consumption. It has the potential to be a valuable biomarker in population studies on the health effects of alcohol, however its stability in long-term stored blood is not known. We used LCMS/MS to assess the stability of PEth-16:0/18:1 in blood samples (packed erythrocytes) that were stored between 1 and 19 years at -80 degrees C in a biobank from a large population survey. The participants answered a lifestyle questionnaire that included questions on alcohol consumption. For analysis, we selected blood samples from seven homogenous ethanol consumption cohorts collected at intervals from 1997 to 2015. Despite the narrow stated alcohol consumption range, 10-15 g/day, there were large differences in PEth values between individuals in the cohorts, from below the limit of detection of 0.005 mu mol/L to 1.40 mu mol/L. The median was 0.08 mu mol/L. Neither generalized linear modeling, nor principal component analysis revealed a statistically significant association between time of storage and PEth levels. The PEth results indicate that the participants had, on average, under-reported their alcohol consumption several-fold. The findings suggest that PEth in blood has a sufficient long-term stability for use as an alcohol biomarker in prospective case-control studies. Analysis of blood stored in biobanks could significantly improve the validity of assessments exploring the health effects of alcohol.