To determine the clinical characteristics of cases of hilar or mediastinal lymph-node cancer in which the primary lesion was not found; reports of 36 such cases were identified. The patients in those cases comprised 29 men and 7 women, whose average age was 58.4 years. The malignancies were found in hilar lymph nodes in 15 patients, mediastinal lymph nodes in 16, and both hilar and mediastinal lymph nodes in 4; 14 were adenocarcinomas, 4 were squamous-cell carcinomas, and 4 were small-cell carcinomas. The average length of cancer-free survival after surgery was 28.9 months, and the maximum was 9 years. After the affected hilar and mediastinal lymph nodes had been resected the clinical appearance of these patients was the same as if the primary tumor had been removed. We therefore believe that many patients given a diagnosis of cancer with metastases to hilar or mediastinaly lymph nodes and in whom the primary tumor cannot be located in fact have primary cancer of the lymph nodes, and that this condition should be called "primary mediastinal lymph-node carcinoma".