Although multiple combinations of weather factors may contribute to an increased incidence of Mycoplasma pneumoniae pneumonia, few studies have investigated the association between weather factors and cases of M pneumoniae pneumonia. Data on cases of M pneumoniae pneumonia and weather factors in Fukuoka, Japan from 1999 to 2007 were obtained and time-series analysis was used to assess the effects of weather variables on M pneumoniae pneumonia cases, adjusting for confounding factors. A total of 13 056 M pneumoniae pneumonia cases were reported during the 9-year study period, of which 12 234 (93.7%) were under 15 years of age. The weekly number of M pneumoniae pneumonia cases increased by 16.9% (95% CI 11.3% to 22.8%) for every 1 degrees C increase in the average temperature and by 4.1% (95% CI 2.7% to 5.5%) for every 1% increase in relative humidity. From 1999 to 2007, cases of M pneumoniae pneumonia increased significantly with increased average temperature and relative humidity in Fukuoka, Japan.