The incidences of temporal arteritis and polymyalgia rheumatica during a twelve year period were studied in different regions of Denmark. Data concerning the incidences of these diagnoses were obtained from two general hospitals from 1982 to 1994 and from the National Patient Register of all diagnoses from all hospitals in 13 of 16 Danish counties from 1982 til 1993. Data from all temporal artery biopsies in two counties were also obtained. Serological epidemiological surveillance data concerning infections causing epidemics in Denmark were obtained from Statens Serum Institut. Data concerning 10,818 patients from 13 counties and 2651 temporal artery biopsies from two counties were analysed. The incidence rate of temporal arteritis in the population aged 50 years and over was 20.4 per 100,000 (95% CI 19-23), and that of polymyalgia rheumatica 41.3 per 100,000 (95% CI 30-67). Significantly higher incidence rates were found in locations with a high population density. The incidence rate of histologically proven temporal arteritis in two counties was 15.1 per 100,000 > 50 years (95% CI 11-20). Pronounced quarterly and annual variations of the incidence were found, with a clustering in five peaks. These cyclic fluctuations were seen simultaneously in several regions. Two periods with an increased incidence of temporal arteritis and polymyalgia rheumatica occurred in close relation to epidemics of Mycoplasma pneumoniae infection. Two peak incidence rates were partly related to epidemics of Parvovirus B19 and one peak to an epidemic of Chlamydia pneumoniae. The synchronous variations in the incidences of temporal arteritis and polymyalgia rheumatica recorded in several regions of Denmark strongly indicate that an environmental infectious factor influences the frequencies. The close concurrence with the above-mentioned epidemics suggests that temporal arteritis and polymyalgia rheumatica may be triggered by certain viral and/or bacterial agents.