Background Socioeconomic mortality differences have increased in many high-income countries in recent decades mainly because of slower mortality decline among the lower social groups. The aim of this study was to investigate whether the changing socio-demographic composition explains the increasing disparity in mortality by income and the stagnation of mortality in the lowest income group.

Methods The register data comprised a nationally representative 11% sample of individuals aged 35–64 years residing in Finland in 1988–2007, linked with mortality records. Household taxable income was used as the income measure. Poisson regression models were used to assess the changes in mortality disparity among the income quintiles between periods 1988–1991, 1996–1999 and 2004–2007. The measures of socio-demographic composition included educational level, social class, employment status and living alone.

Results The mortality rate ratio (with the highest quintile as the reference category) of the lowest quintile increased from 2.80 to 5.16 among the men and from 2.17 to 4.23 among the women between 1988–1991 and 2004–2007. Controlling for other socio-demographic variables strongly attenuated the differences, but the rate ratio of the lowest quintile still increased from 1.32 to 1.73 among the men and from 1.13 to 1.66 among the women. There was no decline in the fully adjusted mortality of the lowest quintiles between second and third study periods.

Conclusions Socio-demographic characteristics explained much of the mortality disparity between income quintiles within each study period. However, these characteristics do not explain the increasing disparity between the periods and stagnating mortality in the lowest quintile.