Hourly precipitation data from 1963 to 1993 of the contiguous United States were analyzed for secular trends in the diurnal cycle of precipitation. During the period the winter daytime and nighttime precipitation frequencies and the diurnal amplitude of precipitation frequency increased by ~30--47% over the Southwest, while the winter precipitation frequencies and the diurnal amplitude of precipitation intensity decreased by similar amounts over the Northwest. During this period, summer afternoon precipitation frequency increased by ~30--60% in the Southwest and decreased by ~15--30% in the Southeast. ¿ 1999 American Geophysical Union