Sporadic cases of legionellosis have increased in Saitama Prefecture. This study aimed to understand the characteristics and incidence of legionellosis in Saitama Prefecture by studying the corresponding data from Tokyo and all over Japan. We analyzed cases of legionellosis registered from 2005 through 2009 in the annual reports of the Infectious Disease Surveillance Center. There were two peaks in the incidence of legionellosis in Japan between June and November, and a trough between February and May every year. Similar seasonal characteristics were observed in both Tokyo and Saitama. Proper management of risk factors-such as cooling towers and other aerosol-generating devices, before and during the seasonal increase in these incidences-is essential as a prophylactic measure against legionellosis.