This paper provides the first data on the incidence of RA based on a prospective population-based register. All new cases of inflammatory polyarthritis in the Norwich Health Authority are notified by general practitioners to the Norfolk Arthritis Register. The patients are then clinically evaluated by metrologists and blood taken for RF estimation. Cases of RA were defined as all those notified with an onset of symptoms in 1990; who presented by 31 December 1991; and who satisfied the 1987 ARA criteria for RA at the time of presentation. Two hundred and ten patients were notified in the defined time-frame, of whom 104 were classified as having RA. The annual incidence rate was 36/100,000 for women and 14/100,000 for men. RA was rare in men aged under 45 yr. The incidence in men rose steeply with age. The incidence in women rose up to age 45 yr, plateaued to age 75 yr, and fell in the very elderly.