Landsat-5 TM images over a forest area in Sweden were calibrated to reflectance. Using images for each summer from 1984 to 1991, changes in the sensor responseoffsetwere found for TM bands 1 to 4. The aerosol optical thicknesses were estimated from horizontal visibility and the sensor deterioration was estimated by assuming a constant lake reflectance. The reflectance changes to be detected in a forest are small compared with the differences between absolute calibrated images. Statistical relative calibration, followed by the scaling of the residuals to reflectance, is therefore recommended for making local reflectance changes comparable over time.