In an oil refinery with a labour force of 1260 male blue-collar workers, 300 matched cases of permanent shift workers, day workers, and drop-outs were selected and split into four groups corresponding in age and years at work. In terms of an overall score ("health score", computed from data concerning absence due to sickness, morbidity, distribution and severity of diseases, and subjective complaints) health was found to deteriorate with age, but to a different degree in the shift and day workers. In shift workers, a steep decrease in score during the first years at work was followed by a continued slight decrease in middle age; from the age of 41 years onwards there was a further pronounced decrease in score. In day workers a stabilization in score was observed up to middle age, with a distinct decrease thereafter. The difference in health parameters between the groups was only small in younger workers (up to 12 years at work), but became striking and significant with increasing age. In the permanent shift workers an increasing health risk was clearly indicated by increases in absence due to sickness, gastro-intestinal and cardiovascular diseases and unspecific health complaints (sleep disturbances, premature fatigue). A specific kind of behaviour during illness (e.g. less readiness to consult a doctor) was also observed in the shift workers. In the permanent day workers health risks were not strictly age-related. Whereas absence due to sickness was highest in young workers, morbidity for respiratory diseases and injuries was significantly elevated in the older workers; the frequency of subjective complaints increased up to middle age and decreased thereafter. In drop-outs with considerable prior exposure to shift work, strikingly high rates of absence due to sickness and excess rates of cardiovascular diseases were observed.