Microscopic examination, particularly by scanning electron microscopy, of the tunica intima of the pulmonary arteries of dogs experimentally infected with a known number of larvae of Dirofilaria immitis revealed various numbers and shapes of intimal thickenings. The number and morphology of these thickenings were related to the degree of infection. At 11 months after infection, dogs having adult worm populations of 8 to 21 worms had scattered discrete intimal lesions, whereas in dogs with 41 to 68 adult worms, the intima was diffusely affected. Discrete lesions took the form of rugous or verrucous protrusions, and in severely affected animals, the intima was extensively covered with anatomosing ridges. Light microscopy and transmission electron microscopy confirmed earlier reports that the thickenings were mainly confined to the tunica intima, that the surface was covered by endothelium, and that many of the cells in the intimal swellings were derived from smooth muscle.