Approximately 60 cases of primary sarcoma of the pulmonary trunk with or without involvement of the pulmonary valve and pulmonary arteries have been reported. Only four cases have been studied by electron microscopy, and all four studies have been based on autopsy material. We have received surgically removed material for electron microscopy in a new case of sarcoma of the left pulmonary artery. The tumor showed an extensive intravascular growth, especially in the branches of the pulmonary artery in the left lower lobe. At the light microscopic level the tumor was predominantly composed of spheroidal cells arranged in a disorganized pattern. The tumor cells had indistinct cell borders and centrally placed, vesiculated nuclei, often with prominent nucleoli. Many tumor cells gave a strong positive reaction for alpha 1-antichymotrypsin. Ultrastructurally the tumor was predominantly composed by light and dark fibroblast-like cells and histiocyte-like cells with ruffled cell borders containing numerous electron dense bodies of lysosomal character. A few undifferentiated mesenchymal cells and intermediate forms were also noticed. The multitude of histological composition of the sarcoma of the pulmonary trunk and arteries reported in the literature favours a derivation from an undifferentiated myointimal mesenchymal cell.