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Objectives: Despite considerable research efforts, reducing the incidence of and work disability associated with back disorders has remained challenging. One of the major reasons is that the natural course of these disorders is poorly understood and therefore identifying new episodes of morbidity remains difficult. The objective of this study was to propose a definition of new onset back disorder for use in occupational studies. Methods: Medical services, hospitalisations and workers' compensation data were linked to create a person-specific, longitudinal database (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) of health care contacts for a cohort of 116 268 workers employed in heavy industry. Contacts with back-related ICD-9 diagnoses were analysed using a semi-parametric mixture model (Nagin, 1999) to investigate trajectories of morbidity over time.Results: During the follow-up period, 22.9% of the cohort had no backrelated health care contacts. Among workers with at least one contact, four back morbidity trajectories provided the best fit of the data. The first trajectory of individuals had a high probability (.60%) of back morbidity every year of follow-up. The other three groups had episodic trajectories of increasing and decreasing probability of back morbidity over time (ranging from 0% to 55% per year). The three episodic groups are postulated to represent the same trajectory with different start dates. Based on health care contacts within these trajectories, a back disorder episode was defined by a minimum of two outpatient visits per year. Separate episodes were defined by a 3-year gap with no health care contacts. Individuals with chronic back disorders were defined by a minimum of four health care contacts per year with no gaps in contacts from year to year. Conclusion: These results provide useful information for defining new episodes of morbidity for use in aetiological studies of occupational back disorders. In this heavy industry cohort, those with no history of back disorders could be followed for future incidence in cohort studies, or could serve as a control group in case-control designs. The episodic disorder group could serve as a source of incident cases in either cohort or casecontrol designs, as long as they had been symptom free for at least 3 years. Key words: back disorders; epidemiologic methods; heavy industry PROXIMAL AND DISTAL DETERMINANTS FOR WORK-RELATED DISEASES: THE EXAMPLE OF LOW BACK PAIN IN THE GENERAL POPULATION IN FRANCEObjectives: In occupational epidemiology distal factors such as level of education are often considered only as confounding factors.1 Our objective was to study the causal pathway leading to low back pain (LBP), taking into account both the proximal factors (physical exposure at work) and the level of education. Methods: In the national survey ''Enquête décennale santé '' conducted in 2002-2003 in France, a self-administered questionnaire included questions on LBP, and past or present exposure to handling of heavy loads and awkward postures...