Background: The purpose of this study was to assess potential differences in the predictive ability of self-assessed health for mortality between educational groups, and to find explanations for any of these educational differences. Methods: We used data from the longitudinal GLOBE study, with a 13-year mortality follow-up. Analyses were performed for people aged between 25-74 years at baseline (n = 16 722). The associations of self-assessed health with mortality were estimated with Cox regression analyses, and the resulting hazard ratios were used as indicators of the 'predictive ability' of self-assessed health for mortality. Differences between educational levels were estimated by including an interaction term of education with self-assessed health in regression models with mortality as the outcome. The analyses were subsequently adjusted for: life threatening chronic conditions, non-life threatening conditions, stressors and health behaviour, to test the contribution of these factors to the predictive ability of self-assessed health. Results: Results indicated that the predictive ability of self-assessed health for mortality was greater in men with tertiary education as compared with the lowest educated men. No differences were observed in women. None of the four health aspects accounted for the educational difference in men. Conclusions: Because differences in the predictive ability for mortality were limited to the extreme educational groups in men, educational differences in self-assessed health that are reported in numerous studies should not be expected to seriously overestimate educational differences in 'objective' health status.