Migration may result in exposure to factors that are both beneficial and harmful for good health. How the act of migration is associated with mortality, or whether the socio-economic condition of migrants prior to migration influences their mortality trajectory, is not well understood. In the present study, a cohort of 413 randomly selected individuals born in the rural community of Överkalix, Sweden, between 1890 and 1935 were followed from birth to either death or old age. Around 50% of the study-population moved away from Överkalix at one time or another. To adjust for a potential bias resulting from self-selection among the migrants, the father's occupational status was used together with parents' and grandparents' longevity. Overall, migration could not be shown to predict mortality when the backgrounds of the migrants were taken into account. Nonetheless, socio-economic background conditions appeared to moderate the association, decreasing the mortality rates for migrants with relatively good pre-migratory socio-economic conditions, while increasing it for migrants with poorer pre-migratory conditions. However, further scrutiny revealed that this effect modification mainly affected the female migrants' mortality. In conclusion, the study suggests that there is no general association between migration and mortality, but that migrants with better socio-economic resources are more likely to improve their mortality trajectories than migrants with poorer resources. Better pre-migratory conditions hence appear to be important for avoiding health-adverse circumstances and gaining access to health beneficial living conditions when moving to foreign environments - especially for women.