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Objective In epidemiological and clinical studies, whole blood assay (WBA) has been used as a measure to characterize inter-individual differences in the cytokine response of individuals exposed to inflammatory agents, such as endotoxins. Several short-time repeatability studies have shown stable cytokine levels in individuals over periods of days, weeks or months, but little is known about the long-term stability of cytokine reactivity. Methods We studied cytokine response levels in LPS-stimulated whole blood in a cohort of 193 farmers and agricultural industry workers at two time points with a five-year interval. Results IL-10 and IL-1β responses measured with a five-year time interval showed a weak positive correlation ( r  = 0.22 and 0.27, respectively), whereas no correlation was observed for TNFα ( r  = 0.06). Cytokine reactivity measured repeatedly at the same time point showed high correlations (IL-10 r  = 0.80, IL-1β r  = 0.53 and TNFα r  = 0.74), suggesting that the observed weak correlations over time are reflective of actual variations in cytokine reactivity over time. Conclusions Repeatability of ex vivo cytokine reactivity showed to be differential for the measured cytokines, being more stable for IL-10 and IL-1β than for TNFα. However, in general, repeatability of ex vivo cytokine reactivity was weak, reflecting that cytokine reactivity can mostly be explained by (short term) intra-individual (immunological) or time varying environmental factors and less by genetic or other time-invariant factors. Therefore, WBA should be regarded as a viable tool to study relationships with current health status and exposure, and only partially as a predictor for a future response. [ABSTRACT FROM AUTHOR]