Fourteen cases (13 pleural and one intrapulmonary) of solitary fibrous tumors (SFTs) (the so-called fibrous mesothelioma) were studied. The lesions occurred more in females (nine cases) than males (five cases). The age of patients ranged from 44 to 73 years old (median 60 years). The tumors presented as cough with or without blood-tinged sputum, exertional dyspnea, chest pain, nausea, body weight loss, fever, or as asymptomatic masses detected by routine chest radiograph. Two patients with huge (tumor larger than 20 cm) malignant tumors had accompanying pleural effusion and one associated with hypoglycemia. Ten benign tumors measured 2-11 cm (median size 7 cm) while the remaining four histologically malignant ones measured 20-30 cm in size. All of them were well circumscribed and thinly encapsulated. Hemorrhage and necrosis were more frequently seen in the malignant tumors. Histologically, these lesions were characterized by 'patternless pattern' with occasional hemangiopericytic features (three cases). The tumor cells were all immunoreactive for vimentin, CD 34, and focally actin-positive in one case, but not for keratin, desmin, S-100 protein, carcinoembryonic antigen, alpha 1-ACT and F VIII-related antigen, supported a primitive mesenchymal origin. p53 protein was expressed in two of the malignant cases. Proliferating cell nuclear antigen stain was positive with 50 and 80% of the labeling index in the benign and malignant tumors, respectively, but retinoblastoma gene protein was negative in all tumors. This analysis confirmed the relationship between histological malignant SFTs and tumor size, cellularity, mitotic activity, necrosis and tumor suppressor gene expression. However, the clinical behavior was unpredictable. Complete respectability seemed to be the most important indicator of clinical outcome in the less aggressive tumors.