The atheroscleroit lesions, associated with the celiac intimal smooth muscle cushions, of four and five year old White Carneau pigeons were studied with the light and electron microscopes. Light microscopic examination of the spontaneous lesions demonstrated large intimal cushions composed of smooth muscle abundant collagen, clusters of foam cells and cholesterol crystal clefts. Ultrastructural examination of the intimal atheroma revealed dilatations between apposing endothelial cells which contained a flocculent material, similar to that seen in the subendothelial space. The subendothelial compartment contained abundant collagen, extracellular lipid, vesiculated material and cell processes which contained a flocculent matrix and tubular-like elements. In addition, fibroblast-like interlaminar cells were often observed. Numerous intimal smooth muscle cells were seen which displayed varied morphology. Abundant foam cells were also present within the intimal atheromas. The presence of atherosclerotic lesions in preexisting intimal smooth muscle cushions suggests that hemodynamic factors may be important in the progression of these spontaneous lesions. Endothelial cell dilatations may provide an important route of transport for circulating elements which may accumulate withing the subendothelial space. Morphologically, it appears that the smooth muscle cells undergo modification and may represent the precursors of foam cells in this species.