Two cases of malignant pleural tumor producing human chorionic gonadotropin, one confirmed at surgery and autopsy and the other by biopsy, are reported. Both of the patients had bilateral gynecomastia with high levels of serum human chorionic gonadotropin. The first patient underwent panpleuropneumonectomy because of diffuse malignant pleural tumor, but died five months later due to recurrent disease. The histological diagnosis was diffuse, malignant, monophasic mesothelioma of the epithelial type. Immunostaining for alpha-, beta-human chorionic gonadotropin and human placental lactogen was positive in syncytiotrophoblast-like cells. The second patient had diffuse pleural tumor with massive effusion. A pleural biopsy specimen showed diffuse proliferation of epithelioid large cells. Immunostaining for alpha, beta-human chorionic gonadotropin and human placental lactogen was positive in mono- and multinucleated bizarre giant cells mimicking trophoblasts. A diagnosis of malignant mesothelioma with trophoblastic differentiation seemed most likely in both cases in view of the clinical and pathological findings. In both cases, results of mucin histochemistry and various immunohistochemical stains for antigens or with antibodies were consistent with diagnosis of mesothelioma except in a few cells in the choriocarcinomatous portion. This may be the first report describing human chorionic gonadotropin-producing malignant mesotheliomas of the pleura.