A climatologic analysis of human rotavirus infection in inpatients with acute diarrhea was conducted over a seven-year period. The infection frequency appeared to be related to temperature, but not to relative humidity. Human rotavirus infection was found to appear abruptly when the mean temperature of any 10-day period became less than 5 C (November or December), reached a peak when it was less than 0 C (January and February), and waned when it became greater than 20 C (June and July) in the city of Yamagata in northern Japan.