Restriction length fragment polymorphism of the L-MYC gene was examined in DNAs from lung cancer tissues and normal tissues of 51 Japanese patients with lung cancer. In individual patients, no difference was seen between the restriction length fragments of the two alleles of L-MYC [6-kilobase (kb) and 10-kb fragments in EcoRI digests] in lung cancer tissues and normal tissues. But a striking correlation was found between the restriction length fragment polymorphism pattern of L-MYC and the extent of metastasis, particularly to the lymph nodes at the time of surgery: Patients with only the L band (10 kb) had few lymph node metastatic lesions, whereas patients with either the S band (6 kb) or the S and L bands almost always had lymph node metastatic lesion. A similar correlation was found between the presence of the S band and metastases to other organs. This correlation was particularly marked in cases of adenocarcinoma. These results indicate a clear genetic influence on metastases and a consequent poor prognosis for certain patients of lung cancer; L-MYC restriction length fragment polymorphism is thus shown to be a useful marker for predicting the metastatic potential of human lung cancer.