A prospective morphological study of tumor involvement of the pulmonary vessels (TIPV) was undertaken on 203 consecutive autopsy cases of malignancies. The lungs were removed as a block and 15 sections (3 from each lobe) were analyzed. Site of origin, histological type and staging of the tumor, topographic distribution of the tumor emboli in the lungs, right ventricular hypertrophy and dilatation, pulmonary infarct, pulmonary vascular sclerosis and lung metastases were recorded in each case. TIPV was detected in 84 (41.4%) cases, the highest frequency reported until now. In 28 cases, TIPV was considered to be the main cause of death. The breast, liver, and pancreas were primary sites in more than 50% of the cases in which TIPV was observed. TIPV was more prevalent in epithelial neoplasms and showed a strong correlation with advanced disease. There was no significant difference among topographic regions of the lungs. The cases with TIPV were correlated with a high frequency of right ventricular hypertrophy and dilatation, vascular sclerosis and pulmonary metastases but not with pulmonary infarcts.