Several studies have recently reported that social class differences in ill-health during adolescence are almost non-existent or invisible. The aims of this comparative study of two different welfare states are first, to compare whether the relationship between social class and health is similar among young men and women at different age groups in these two welfare states; second, to examine at what age social class differences in self-reported health and illness among young adults emerge in these two countries; and third, to find out whether class of origin (i.e. parental social class) or class of destination (i.e. individual's achieved social class) have greater explanatory power in studies of health among young adults. We used comparable nationally representative interview surveys from Britain and Finland. The British data is derived from the General Household Survey for 1988 and 1989, and the Finnish data from the 1986 Level of Living Survey. We analysed five year age groups between 16 and 39 years in Britain (N = 16,626) and 15 and 39 years in Finland (N = 5950). Two health indicators (limiting long-standing illness and self-assessed health), and several indicators for social class were compared. The best discriminator of differences in ill-health among young adults both in Finland and Britain was education. Social class differences by own occupation (achieved class) emerged soon after the age of 20 among men and women in both countries and strengthened with increasing age. There was a weaker but consistent association with class of origin in both countries. Housing tenure is strongly associated with young adults' health in Britain but not Finland.(ABSTRACT TRUNCATED AT 250 WORDS)