Fluctuations of temperature in the Rocky Mountain states of Idaho, Montana, Wyoming, Colorado and Utah are analyzed for the period 1891–1978. Prior to 1891, data for this area are too sparse to make meaningful generalizations. After screening temperature data for homogeneity, selected records are used to map seasonal mean temperature departures from 1941–70 means. In all seasons, mean temperatures were higher by 1–1.5°C during the last 40 years compared to the previous 40–50 years. However, spring and fall temperatures have been declining since the period maximum in the late 1930's. Pentad mean winter and summer temperatures show a statistically significant temperature increase throughout the entire period, temperatures rising by 0.09 and 0.1°C per decade, respectively. In all seasons, except winter, the warmest decade was the 1930's. Winters were warmest in the period 1906–10 when summers were relatively cool. The coldest decade was centered around 1910–15 though for winters the period 1891–1900 was colder. An inverse relationship was found between decadal temperature and precipitation departures from 1941–70 averages; this may reflect a contrast between periods characterized by a high frequency of warm, dry anticyclonic conditions and periods with a higher frequency of cool, moist cyclonic activity. Growing season lengths were ∼15% shorter around the beginning of this century compared to the 1930's as a result of both later spring and earlier fall killing frosts. Heating season degree day totals from the 1880's to the 1920's averaged ∼140 degree days (°C) more than in 1941–70 over the whole area. This is estimated to be equivalent to an increase in the space heating load of ∼1 barrel of oil per standard house per year.