The anatomy of tumor wood and normal wood in a branch of Korean red pine (Pinus densiflora S. et Z.) is compared. In tumor wood the tracheids and rays are irregularly arranged, the cell lumina contain resinous substances and fungal hypha, the tracheid files are of variable width within the growth ring, and traumatic vertical resin canals are present. There are also differences in length of tracheids, numbers of vertical and horizontal resin canals (fusiform rays), height of fusiform rays, and number and height of uniseriate rays. The modified structure of the tumor wood originates near the pith. The wider growth rings, considerably shorter but not wider tracheids, and larger and more numerous rays in tumor wood are associated with a higher radial growth rate, which in turn results in the externally massive wood tumor.