The prognosis in rheumatoid arthritis respecting mortality was studied in a consecutive series of 489 hospital patients over a period of 18 years. The relative risk of mortality was raised in both men (2.6; p less than 0.001) and women (3.4; p less than 0.001). In the women the relative risk was also influenced by prior duration of RA and was characterised by a diminution in risk 5-9 years after first presentation. Relative risks for men were more uniformly distributed over time. Annual excess mortality rates were strongly associated with age at first presentation in women, the rate increasing with increasing age in both the group seen within 5 years of onset of disease (chi 2(1) for trend = 30.4; p less than 0.001) and in the later referral group (chi 2(1) = 34.0; p less than 0.001). A similar but much less marked effect was observed in men in the early referral group (chi 2(1) = 13.7; p less than 0.001) only. These results suggest that initially women may have a milder form of disease and that hormonal status may affect prognosis. Future long-term therapeutic studies in RA should take into account the prognostic factors of age, sex and duration of disease.