Numerous studies have demonstrated associations between psychosocial stress and indices of poor health, and much research is now dedicated to identifying the responsible biological mechanisms. The current study examined the hypothesis that stress may impact health by promoting immunesenescence. Participants were 537 factory workers (89% male; mean age 44; range 18-65years). Blood was analyzed for two components of the aging 'immune risk phenotype': the number and proportion of late-differentiated (CD27-CD28-) CD8 T cells (CTLs) and CD4:CD8 ratio. Psychological assessment focussed on work-related stressors which have previously been found to predict morbidity and mortality. This assessment included measures of work load, effort-reward imbalance, and social support at work. High levels of job stress (low reward, high effort-reward imbalance) and low social support at work were associated with a significantly lower CD4:CD8 ratio. Also, the number of CD27-CD28- CTLs was 30% to 50% higher in employees classified in the highest tertile of each stress parameter as compared to employees in the corresponding lowest tertile (p<.01). These associations withstood adjustment for a wide range of demographic, life style, medical, and socio-economic indicators. The associations between CTL phenotype and low social support became stronger with increasing age. These results suggest that psychosocial stress may contribute to immunological aging. Prospective studies should address the long-term consequences of these associations for healthy aging.