Continuous decline in functioning is not an inevitable consequence of ageing, as some individuals maintain high levels of functioning to old age. The origins of functional problems in old age are not only related to current circumstances, but may be traced back to earlier life-experiences. Previous analyses show that change in functioning is related to socioeconomic status, but it is uncertain to what extent these differences can be accounted for by the same health behaviours and material and psychosocial factors that are related to socioeconomic differences in morbidity and mortality. This paper investigates socioeconomic differences in change in mental and physical functioning in a relatively young and healthy population over a three year follow-up period. The data come from the Whitehall II Study of London based civil servants aged 39-63 in 1991-93. We measured functioning with the Short Form 36 (SF-36) General Health Survey and socioeconomic status by civil service employment grade. Among lower employment grade men the odds ratio of being in the quartile of rapid decline in mental and physical functioning was 1.79 and 1.56 respectively. For women the odds ratio for physical functioning was 1.34, but employment grade differences in mental functioning were inconsistent. Among men health behaviours were the most important determinants of employment grade differences in physical functioning change. In addition, material problems and job decision latitude accounted for employment grade differences in physical as well as mental functioning change. However, among women employment grade differences in change in physical functioning can not be accounted for with these risk factors. Analyses of change in determinants may provide further insight into the underlying pathways. Early detection of functional decline and intervention may be a key to better functioning in ageing populations.