Respiratory tract infections in children, measured in terms of both morbidity and mortality, represent a major health problem in Greenland. In particular, otitis media is highly prevalent, and is characterized by early onset and a high degree of chronicity. There is, however, little knowledge about the epidemiology of respiratory tract infections in Greenland. In the spring of 1996, a prospective study of such infections in infants and children was initiated in Sisimiut, West Greenland. The main objectives of the study are to describe the epidemiology of respiratory tract infections in children under four years of age, to estimate the impact of these diseases on short- and long-term morbidity, and to identify risk factors for transmission and clinical severity. An open cohort of children in Sisimiut will be formed, including all resident children below three years of age at the beginning of the study, as well as all children born in the following two-year period: in total approximately 600 children. During two years, these children will be followed closely, including registration of episodes of respiratory tract infections. Furthermore, growth will be measured and microbiological samples obtained.