Objective: To measure within-person change in scores on the short form general health survey (SF-36) by age, sex, employment grade, and disease status. Design: Longitudinal study with a mean of 36 months (range 23–59 months) follow up, with screening examination and questionnaire to detect physical and psychiatric morbidity. Setting: 20 civil service departments originally located in London. Participants: 5070 male and 2197 female office based civil servants aged 39–63 years. Main outcome measures: Change in the eight scales of the SF-36 (adjusted for baseline score and length of follow up) and effect sizes (adjusted change/standard deviation of differences). Results: Within-person declines (worsening health) with age were greater than estimated by cross sectional data alone. General mental health showed greater declines among younger participants (P for linear trend Conclusions: Health functioning, as measured by the SF-36, changed in hypothesised directions with age, employment grade, and disease status. These changes occurred within a short follow up period, in an occupational, high functioning cohort which has not been the subject of intervention, suggesting that the SF-36 is sensitive to changes in health in general populations. The SF-36, an inexpensive measure of health outcomes, is capable of detecting change in health in a general population Health and functioning do not decline uniformly with age; general mental health shows greater declines among younger participants Socioeconomic status is associated inversely with baseline functioning and, independently, with decline in health The greatest declines were seen among subjects with physical and psychiatric morbidity at baseline Performance of 28 doctors and medical laboratory scientific officers in distinguishing pairs of slides