Fever is a common and important clinical symptom observed among hospitalized geriatric patients. To investigate the frequency and duration of fever episodes, we surveyed fever episodes in a hospital where the frequency of patients over 60 years of age exceeds 90 per cent of the patients. Fever episodes with body temperature of over 37.5 degrees C were registered from May in 1991 to December in 1994, and 6809 episodes were subjected to analysis. The average incidences per month were 157.1, 165.3, 158.0, and 139.3 in 1991, 1992, 1993, and 1994, respectively. The numbers of episodes per month did not show any significant correlation with temperature or humidity. Average duration of the episodes were 8.0, 6.5, 7.6, and 6.7 days for 1991, 1992, 1993, and 1994, respectively. Episodes of one day duration were the most frequent in all months, and the frequencies of that ranged from 37.1% to 58.6% with a mean of 47.8%. The average duration of episodes and the frequency of one day episodes did not change significantly irrespective of a notable decrease in the total incidence. The high frequency of one day episodes and their consistency through the observed period suggest that fevers with one day duration are one of the characteristic features of the febrile symptoms in geriatric patients. Causality and prevention methods for these one day fever episodes should be investigated.