Background Workers' organisations may be seen as an opportunity to reduce levels of work stress and its adverse health impacts. One relatively under-studied stress-related health outcome is Irritable Bowel Syndrome (IBS) – a condition with high prevalence in the working population.

Objectives We evaluated whether social groups with a high membership in labour organisations (labour organisation density – LOD) influenced the prevalence of IBS among Chilean workers. We hypothesised that there is a protective effect of LOD on IBS prevalence and that this protection acts at the group level when workers can collectively exert more control over their working lives.

Methods The study design was cross-sectional, based on data from stratified random national surveys conducted by the Chilean Department of Labour and the Department of Health in 2009–10. Generalised linear multi-level models (GLMM) were „constructed using IBS prevalence as the outcome (Poisson distribution; log link), and LOD as the main predictor processed with different cutoffs. LOD was assessed in occupations and regions which constituted the group level in multi-level models. Prevalences and prevalence ratios (PR) were calculated.

Results GLMM models showed adequate fit; several cutoffs defining low versus high LOD were investigated. When the LOD cutoff was 17%, the IBS prevalence was 6.0% compared with 4.3% for higher LOD (p = 0.04). This association held for different definitions of the grouping variable (Occupation/Region) and for alternative cut points of LOD.

Conclusions Occupations/Regions with a higher percentage „participation in labour organisations had lower prevalence of IBS. Studies with information on labour participation and other predictors of work stress at both the individual and group levels are needed to make a stronger case for causality of this association