Dieback and mortality of Alnus incana subsp. tenuifolia in the Southern Rocky Mountains apparently began by the late 1980s and have become a concern to land managers. A survey of alder including 68 transects from southern Wyoming to northern New Mexico indicated that, of 6,503 standing stems, 37% were dead, 29% had dieback, and 34% were healthy. Transects intercepted 1,479 m of live and 1,177 m of dead alder canopy. A second, more localized survey with 32 transects in the upper Gunnison River watershed of Colorado yielded similar results. Abundance of live sprouts was inversely related to amount of dieback and mortality in a genet, suggesting that affected genets are dying and not replacing themselves. Damage did not vary substantially by geographic area and was not related to elevation, animal browsing, or distance to nearest road. Distance to nearest stream was weakly, inversely related to severity of dieback and mortality. Symptoms were not consistent with disease of alder caused by Phytophthora alni in Europe, and isolations for Phytophthora species were negative. Cytospora canker, caused by Valsa melanodiscus (anamorph Cytospora umbrina), is the proximate cause of the dieback and mortality.