Pulmonary blastomas. Pulmonary blastoma is a rare lung tumor composed of immature mesenchyme and/or epithelium that morphologically mimics embryonal pulmonary structure. The prognosis of these tumors is poor, and the clinical course is not readily predicted from histologic appearance. In this report, the clinical, gross, microscopic, and immunopathologic features of 52 cases are described, and prognostically important correlates are determined. Twenty-eight patients were women, and 24 were men. There was a unimodal age peak in the fourth decade; only two patients were younger than 10 years old, and both had biphasic blastomas. Forty-one percent of patients were asymptomatic. Chest radiography typically showed a peripheral or midlung mass without predilection for any lobe. Microscopically, tumors could be divided into two classes: those composed solely of malignant glands of embryonal appearance (well-differentiated fetal adenocarcinomas [WDFA], 28 cases) and those with a biphasic appearance (24 cases). The malignant epithelium contained cytokeratin, carcinoembryonic antigen, milk fat globulin, and often chromogranin; vimentin, actin, and less frequently desmin and myoglobin were present in malignant stromal cells. More often WDFA was a smaller tumor (less than 5 cm) than biphasic tumors (P less than or equal to 0.001). It was more likely to be asymptomatic (P less than or equal to 0.001), and it was less likely to show pleural effusion by chest radiography (P less than or equal to 0.01) or giant or bizarre tumor cells (P less than or equal to 0.001) or frequent (greater than or equal to 30 mitoses/10 high-power fields) mitoses in the microscopic sections (P less than or equal to 0.01). Only 14% of patients with WDFA died of their tumor; 52% of patients with biphasic tumors died (mean follow-up, 97 months and 49 months, respectively). For patients with WDFA, the presence of thoracic adenopathy by chest radiography (P less than or equal to 0.001) and metastasis at initial presentation (P less than or equal to 0.001), followed by tumor recurrence (P less than or equal to 0.01), were the factors most highly correlated with poor prognosis. For patients with biphasic tumors, tumor recurrence (P less than or equal to 0.001) was the most significant indicator of poor prognosis, followed by metastasis at initial presentation (P less than or equal to 0.05) and gross size of the tumor (greater than or equal to 5 cm) (P less than or equal to 0.05). These findings support the idea that histologic class and gross and clinical findings can be of prognostic value in pulmonary blastoma. 