PURPOSE: Chronic sequelae (CS) are increasingly important to drinking water risk assessment and management processes, but there has been relatively little scientific rigor in defining CS or reviewing the literature on water-related CS. Our purposes were to develop a scientific definition for CS and to evaluate the definition's practical merit.METHODS: We examined scientific publications for definitions of "chronic sequela." We developed a definition that is based on scientific concepts and that can be systematically applied to literature to assess whether pathogen-related health outcomes qualify as CS. As a case study, we conducted an extensive Medline search and tested our definition on the epidemiological and clinical literature linking Campylobacter and GBS.RESULTS: We defined "chronic sequela" as the secondary adverse health outcome that 1) occurs as a result of a previous infection by a microbial pathogen, and 2) is clearly distinguishable from the health events that initially result from the causative infection, and 3) lasts 3 months or more after recognition. The 12 Campylobacter and GBS studies (five epidemiological and seven clinical) revealed that current data reporting practices limit the evaluation of all three elements in our definition. Laboratory methods and criteria to characterize infection were not always adequately reported. Primary and secondary health events were always reported, but eight of the studies required obtaining additional articles to determine the GBS criteria used. Ten of the 12 articles contained duration data for the GBS symptoms.CONCLUSIONS: Much of the evidence needed to apply our definition was found in the studies reviewed, but changes in reporting practices would facilitate the scientific evaluation of pathogen-CS relationships and estimation of their public health magnitude.