The objective of this prospective cohort study was to determine the contribution of changes in social class within and between generations to mortality risk and to socioeconomic differentials in mortality. In 27 workplaces in the west of Scotland. 5567 men aged 35-64 years when screened, provided information on their father's occupation, their own first occupation and their occupation at screening. Mortality risk, from broad causes of death by intergenerational and intragenerational social mobility groups, was measured after 21 years. For all or some of the 3 routes of mobility (childhood to screening, labour market entry to screening and childhood to labour market entry), increasing values were seen across the 4 groups (stable non manual, upwardly mobile, downwardly mobile and stable manual) for diastolic blood pressure, body mass index, current smoking, early school leaving, angina, bronchitis, severe chest pain, and proportion living in deprived areas. Decreasing values were seen for serum cholesterol, height, FEV1, exercise, never and ex-smokers, wine drinkers and car users. For mobility between childhood and screening and between childhood and labour market entry, mortality risk was highest for the stable manual group and lowest for the stable non manual group for all cause, cardiovascular disease and other causes of death. The upwardly and downwardly mobile groups had intermediate risks. For cancer mortality, the stable manual group had the highest risk with the other groups having lower and similar risks. For mobility between labour market entry and screening, the highest risk was for the downwardly mobile group for all cause and cardiovascular mortality. For cancer mortality, the risk was higher for men in manual social classes at all occasions. Adjustment for risk factors attenuated but could not completely explain the differentials in mortality risk. Overall, major differences in mortality risk were seen between the stable non manual and the stable manual groups, to which social mobility does not contribute. With the exception of the small intragenerationally downwardly mobile group there was little evidence that social mobility itself was associated with mortality outcomes different from those expected on the basis of lifetime socioeconomic experience. This is consistent with the suggestion that the main influence of socioeconomic position on mortality risk is an accumulative one, acting across the lifecourse.