The occurrence of giant cell arteritis (GCA) (considering temporal arteritis and polymyalgia rheumatica as its manifestations) was investigated in Western Nyland, Finland. Between 1984 and 1988 54 patients were diagnosed in a population of about 44,500. Fourteen patients (26%) had biopsy proven arteritis at the initial evaluation, and 2 patients became biopsy positive during followup. The retrospectively determined annual incidence of GCA in a 44 month period was 22.5/10(5) in the whole population, and 69.8/10(5) in residents aged 50 years or older. The corresponding figures of a prospective 16 month study were 30.4/10(5) and 94.4/10(5). The incidence rates of the prospective study are higher than those reported in comparable studies. Polymyalgia rheumatica, occurring alone or combined with cranial symptoms, was the most frequent manifestation of GCA, being present in 87% of patients.