The 18-year data base for vertical column abundance measurements of atmospheric OH at Fritz Peak Observatory, Colorado (40 ¿N, 105 ¿W), exhibits a relatively abrupt change in seasonal behavior during 1991 through 1995. Late summer through early fall abundances, especially, for each year of the 1991--1995 period are significantly below the comparable seasonal average for 1980--1990. Early morning abundances for August through October at solar zenith angles greater than 45¿ exhibit an average reduction of about 0.8¿1013/cm2, which at a solar zenith angle of 60¿ is a 15% column decrease. ¿ American Geophysical Union 1996