The aim of this study was to analyse sex differences in medically certified sickness absence with special regard to parental status and age. All new sick-leave spells exceeding seven days and certified by a physician in a county of Sweden were registered in 1985-1987. The data-base was related to the population at risk by using data from the Swedish Official Population and Housing Census 1985. The analysis concerned the employed population in the age group from 16 to 54 with and without custody of children younger than ten years. In all ages, women with children had more sickness absence than men with children. Corresponding sex differences did not appear for persons without children, except for the oldest age-groups. Men with children had the lowest sickness absence of all groups. Young women with children had the highest. These findings remained the same when pregnancy-related sickness absence was excluded. The common finding of sex differences in sickness absence is here highly interrelated to custody of small children. The results suggest that the often reported positive health effects of multiple roles in women may be counteracted by inequality between the sexes in the responsibility as parents.