Patterns of branch growth in Pinus contorts Dougl. ex Loud, (lodgepole pine) on the east side of the Cascade Range in Oregon were significantly altered by Arceuthobium americanum (lodgepole pine dwarf mistletoe). There were decreases in the number, length, and mass of needles, as well as in the length and mass of twigs. These reductions were correlated with the infection status of individual branches. Generally, twigs from uninfected branches supported the greatest number, size, and mass of needles, as well as the greatest twig mass. Twigs from branches having localized infections were intermediate for these same characteristics, whereas twigs from systemically infected branches were lowest. These differences suggest that changes in metabolic function of the host result from infection by dwarf mistletoe. The changes are probably among the factors that contribute to host decline correlated with increases in severity of infection.