We studied the epidemiology of giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) in a Mediterranean population. Ninety-nine patients with PMR and/or GCA were identified over a 9-year period (1980-1988) in Reggio Emilia, Italy. The average annual incidence of PMR and GCA was 12.7/100,000 and 6.9/100,000, respectively, in a population aged 50 years or older. Frequencies of HLA antigens were determined in 49 patients with PMR and/or GCA who were followed by staff at our rheumatology unit during the 1980-1988 period. When compared with HLA findings in 242 healthy controls, DR4 was not found to be significantly associated with PMR (24% in PMR patients versus 14% in controls). Patients with GCA also showed an increased frequency of DR4 compared with controls (36% versus 14%), but this difference was also not statistically significant. The immunogenetic features of PMR and GCA and the relationship between the immunogenetic and epidemiologic patterns in different populations are discussed.