The annual incidence of giant cell arteritis (the term used in this study to encompass the syndromes of temporal arteritis and polymyalgia rheumatica, occurring either together or alone) was prospectively determined in a Danish county that had a population of approximately 200,000. In a single year, 46 new cases of giant cell arteritis were diagnosed, a number which corresponds to an incidence in the overall population of 21.5/10(5), and to an incidence of 76.6/10(5) for individuals age 50 years or older. These rates are higher than those previously reported in retrospective studies. The 3-year followup of all patients showed no onset of other diseases that would require a revision of the original diagnosis. There was no deviation from the age- and sex specific malignancy rate or the mortality rate in the overall population. Women had an incidence rate 4 to 5 times higher than that seen in men. Symptoms, for the most part, were the same as those found in other studies; however, vision loss was not observed during the followup period. Point prevalence at the start of the study was 37.8/10(5), which is below the rates previously reported. This is probably because of failure on the part of participating physicians to record all cases.