The paper uses data from a sample of 1 percent of the male population of England and Wales to examine the contribution of social mobility between the censuses of 1971 and 1981 to socioeconomic differences in health. Compared to others in their social class of origin, men who had been downwardly mobile were more likely, and the upwardly mobile were less likely, to report a limiting long-term illness. However, when compared to others in their classes of destination, those who moved down reported less illness, and the upwardly mobile reported more. Prevalence of ill health in mobile men was somewhere between that in the group they left and the group they joined. Social mobility was a common event and, combined with existing socioeconomic differences in health, it acted to constrain rather than to increase these differences.