The purpose of this paper is to describe seasonal fluctuations in hospitalisation for pneumonia in Finland over the period 1972-1993. Treatment periods with pneumonia as the main or secondary diagnosis were collected from the national hospital discharge register. The seasonality of monthly hospital admissions was analysed using the X11 ARIMA procedure. The population of Finland numbered 5 million during the period concerned, and there were a total of 453,393 pneumonia treatment periods, 17.9% in the age-group under 16 years, 29.6% among working-age persons and 52.5% among persons aged over 64 years. The number was greatest in the total population in December (20.5%) and January (25.1%) and lowest in July (-19.7%) and August (-24.3%). Seasonal fluctuations in pneumonia hospitalisations pointed to the same trend in both the sexes, in that the number increased in winter and decreased in summer. Children's admissions began to increase a month earlier than those of the other age groups, however, in November, while a transient increase in admissions of working age persons in March is attributable to the more extensive use of hospital services by army conscripts. Admissions of persons aged over 64 years fell more slowly from the winter peak than did those of the other segments of the population. The winter peak in hospital admissions for pneumonia can be anticipated administratively by increasing the number of hospital beds and intensifying outpatient care. It may not be necessary to increase health care resources to achieve this, however, as the necessary resources can be obtained through reductions in the summer months. Proper recognition of admission peaks, the points in time at which these regularly occur and the target groups within the population can contribute greatly to health care planning and its economy.