Objectives Low socioeconomic position is consistently associated with higher rates of sickness absence. We aimed to examine whether working conditions, health-related behaviours and family-related factors explain occupational class differences in medically certified sickness absence.

Methods The study included 5470 women and 1464 men employees of the City of Helsinki, surveyed in 2000–2002. These data were prospectively linked to sickness absence records until the end of 2005, providing a mean follow-up time of 3.9 years. Poisson regression was used to examine the occurrence of medically certified sickness absence episodes lasting 4 days or more.

Results Medically certified sickness absence was roughly three times more common among manual workers than among managers and professionals in both women and men. Physical working conditions were the strongest explanatory factors for occupational class differences in sickness absence, followed by smoking and relative weight. Work arrangements and family-related factors had very small effects only. The effects of psychosocial working conditions were heterogeneous: job control narrowed occupational class differences in sickness absence while mental strain and job demands tended to widened them. Overall, the findings were quite similar in women and men.

Conclusions Physical working conditions provided strongest explanations for occupational class differences in sickness absence. Smoking and relative weight, which are well-known determinants of health, also explained part of the excess sickness absence in lower occupational classes. Applying tailored work arrangements to employees on sick leave, reducing physically heavy working conditions and promoting healthy behaviours provide potential routes to narrow occupational class differences in sickness absence.