time series of temperature and rainfall in northwestern ontario are joined and adjusted to provide a database for analysis of regional climate change. processes used to locate and adjust for non-climatic discontinuities are provided. temperature trends were computed for 1916-1998. all available stations have increases in mean annual temperature. the greatest warming occurred in the spring at all stations with most stations > 1.0° c warmer. some increase in winter and the growing season temperature has taken place. a slight negative change has taken place in the fall in most stations. the warming has been greater in minimum temperatures than maximums with resulting declines in the diurnal temperature range especially evident in the first half of the period. average rainfall has increased at all stations in the region. an analysis of daily rain events suggests increases in frequency and amounts of individual events >40 mm in southern locations and 30.0 - 39.9 mm in the northern stations. trends and variability in these variables during the 20th century are analysed to define a base and context for predictions of significant climate change in the 21st century. ecosystem change because of climate change is likely to have major impacts on forest management and wood-based products, an economic sector of major importance in the region.