OBJECTIVE: This study examined whether the number of emergency department visits for "mental and psychosocial problems" varies with temperature or humidity. METHODS: The number of visits in three geographic areas of Quebec were examined as a function of temperature and humidity by using routinely collected May-September data for 1995-2007 (NEuro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin)347,552 visits). Data for two age groups (under age 65 and age 65 and older) were examined. Incidence rate ratios for mean temperature and humidity were estimated by using Poisson regression and generalized additive models. RESULTS: The number of visits tended to increase with increasing mean temperature. At 22.5 degrees C (72.5 degrees F) and 25 degrees C (77.0 degrees F), the number was usually significantly higher than average. Visits increased with humidity in the younger age group. CONCLUSIONS: Results suggest increased use of emergency departments for mental and psychosocial problems with higher mean temperature and humidity, especially in metropolitan areas and in southern Quebec. Climate change may make this effect increasingly important.