Pulmonary adenocarcinoma composed of pure or predominant solid components is reported to be a highly malignant tumor. However, the existence of solid components and its connection with biological behavior have not been well documented. To answer this question, we histologically subclassified poorly differentiated adenocarcinoma (P/D Ad Ca) into solid type and non-solid type, and compared the biological behavioral characteristics.Material and Methods: All histological specimens of surgically resected primary lung carcinoma diagnosed as P/D Ca or large cell carcinoma in Nippon Medical School Hospital were re-evaluated according to the 1999 WHO manual. The cases re-evaluated as P/D Ad Ca were further divided into solid type and non-solid type according to our original definition: the solid type contains solid components where a glandular structure is not recognized in more than one high-power field, while in the non-solid type, a small glandular structure is observed in every high-power field. The differences in the occurrence of lymph node metastasis were assessed by Fisher's exact test.Results: Among 109 cases satisfying both histological and clinical investigation, 45 cases were re-evaluated as P/D Ad Ca; solid type (n=22), and non-solid type (n=23). Lymph node metastases occurred at a higher rate in the solid type than in the non-solid type (p<0.01).Conclusion: Patients with solid type Ad Ca have reached a more advanced stage than patients having non-solid type due to high metastatic rate to lymph nodes. These results suggest that we should not overlook solid components even if the solid components are the focal lesion. This sub-typing alerts clinicians to survey metastases, and may contribute to therapeutic strategies in the future.