Advanced Very High Resolution Radiometer (AVHRR) data from the National Oceanic and Atmospheric Administration (NOAA)-9 satellite were acquired of the western United States from March 1986 to November 1987. Monthly maximum value composites of AVHRR normalized difference vegetation index (NDVI) [(near infrared - visible)/(near infrared + visible)] were calculated for 19 coniferous forest stands in Oregon, Washington, Montana, and California. The leaf area index (LAI) of the conifer forests explained 70% and 79% of the variation of the summer maximum AVHRR NDVI in July 1986 and July 1987, respectively. The seasonal variation of NDVI was related to phenological changes in LAI, as well as the proportion of surface cover types contributing to the overall reflectance. The varying solar zenith angles in the summer and winter months complicated analyses of the seasonal differences in LAI of the forest stands by reducing NDVI values in the winter months. It is concluded that AVHRR NDVI data from July were related to the seasonal maximum leaf area index of coniferous forests of the western United States, and that seasonal differences in the AVHRR NDVI were related to: a) phenological changes in LAI caused by climate, b) the proportions of surface cover types contributing to the overall reflectance, and c) large variations in the solar zenith angle.