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 < 0.001). Employment grade was inversely related to change; lower grades had greater deteriorations than higher grades (P < 0.001 for each scale in men; P < 0.05 for each scale in women except general health perceptions and role limitations due to physical problems). The greatest declines were seen among participants with disease at baseline, with the effects of physical and psychiatric morbidity being additive. Effect sizes ranged from 0.20 to 0.65 in participants with both physical and psychiatric morbidity. 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.