EcoRI restriction fragment length polymorphism (RFLP) of the L-myc gene was examined in leukocyte DNAs isolated from 381 breast cancer patients. No differences in the patterns of L-myc RFLP were found between breast cancer patients and healthy individuals. However, among 97 patients who relapsed, a statistical correlation was found between L-myc RFLP and lung metastases (p less than 0.05). These results are in close agreement with previous findings in patients with cancer of the lung, bone or kidney, and suggest that L-myc RFLP may be a useful marker for predicting lung metastasis in some human cancers.