Two cases of solitary fibrous tumor of the pleura with features of malignancy are described. In the first case, the tumor macroscopically showed noncircumscribed growth. Microscopically, on low power examination, the tumor was characteristically „patternless“, with alternation of cellular areas and hypocellular, prominently collagenized areas. There was an infiltrative growth present at the margins. Cytological atypias were not present. In the second case, the tumor was macroscopically circumscribed. Microscopically, on low power examination, the tumor had characteristical „patternless“ appearance again. Pleomorphic cells with high mitotic activity dominated in cellular areas on high power examination. The infiltrative pattern of growth was not present at the margins. Both tumors were classified as malignant solitary fibrous tumors of the pleura, or fibrosarcomas of the pleura. The criteria of malignancy for solitary fibrous tumor are discussed.

        Key words: solitary fibrous tumor – malignancy