Objective: Some individuals will not meet criteria for a lifetime alcohol use disorder (AUD) at a baseline assessment but will at a follow-up measurement, but not because the disorder began after the initial evaluation. Despite several research implications, this type of unreliability of lifetime AUD estimates has not been studied extensively. The present study investigated the extent of false negatives in the assessment of lifetime AUDs using longitudinal data. Method: A prospective cohort of college freshmen (baseline N = 489) were assessed seven times between ages 18 and 34 years using the Diagnostic Interview Schedule. Individuals were categorized as false negatives at the index assessment using a retrospective (using Diagnostic and Statistical Manual of Mental Disorders, Third Edition [DSM-III], and DSM-IV data), a prospective (using DSM-III data only), and a combined approach (using DSM-III data only). Results: For DSM-IV, of the 29 ostensible new onsets at a follow-up 5 years later (age approximately 34 years), 28 (96%) reported meeting AUD criteria before the index assessment (age approximately 29 years). For DSM-III, of the 25 ostensible new onsets, the retrospective, prospective, and combined approaches categorized 18 (72%) individuals as false negatives at the index assessment. Conclusions: These fi ndings further demonstrate sensitivity issues with lifetime AUD assessments and call into question the validity of "onset" cases that rely on only two waves of data, especially when the follow-up assessment fails to reassess lifetime fully (i.e., across the entire drinking history).