In all industrialized societies health status in adults has been found to vary with social position. Attempts to explain this are usually grouped under headings of artefact, material, lifestyle and selective mobility of the healthiest. Such attempts have to date been unsuccessful in fully accounting for this relationship, and whilst they have merit have left unconsidered the effects of the process whereby social stratification occurs. The present study is a prelude to subsequent studies that will endeavour to distinguish between three separate influences on health--the effects of current social position, the long term effects stemming from one's initial class position, and the effects of the processes governing mobility. The purpose of our present investigation is to describe patterns of occupational mobility, that will enable us to identify possible predictors of subsequent mobility and therefore to indicate to what extent mobility might be a process governed by social rules. The work presented in this paper comprises part of the Whitehall II study of occupational, social and lifestyle influences upon health in a Civil Service population. Using multiple regression techniques almost half the variation in mobility is modelled in terms of educational level, fathers' social class, gender, marital status, age on entry into the Civil Service, length of time in Civil Service employment and grade of entry into the Civil Service. Using estimates derived from this model it is suggested that a number of sub-groups within the Civil Service suffer adverse mobility (mobility appears particularly restricted for women and for those entering the Civil Service above 30 years of age). The results obtained suggest that the issue of obstructed opportunity at the workplace could become a focus for fruitful investigation, linking issues of personal autonomy, expectations and control to health. A number of methodological problems in this kind of work are considered together with discussion of how the model can be used to increase our understanding of mobility.