Seven years ago, a cluster of severe bronchiolitis obliterans cases among former workers of a small rural microwave popcorn plant precipitated the detective work which identified the hazard of volatiles derived from butter flavoring. In the initial report, diacetyl, a diketone which imparts buttery aroma and flavor to foods, was described as a marker of flavoring exposure. Diacetyl exposure was associated with abnormal lung function, decreased forced expiratory volume in one second (FEV1), and the mixer job title that was subsequently shown to be highest risk in industry-wide investigation. The same severe fixed obstructive syndrome in many flavoring manufacturing workers substantiates the hazard of diacetyl, and biologic plausibility now exists in rodent toxicology studies. In the current issue of the Journal, van Rooy and coauthors make several substantial contributions to the understanding of this newly recognized occupational hazard to flavoring-exposed workers. First, diacetyl manufacture produced at least four cases of severe bronchiolitis obliterans syndrome among 103 process operators in an historical cohort, including the post-study case found in one of the ten nonparticipants. Second, none of the four cases had been recognized as bronchiolitis obliterans or as occupationally related, which is typical of the cases being found throughout both microwave popcorn and flavoring manufacturing industries.