The L1 molecule has emerged as a promising new biomarker for the diagnosis and prognosis of human ovarian and endometrial tumors. It was initially described as an adhesion molecule for neural cells but its function on tumor cells is less well known. In this article, the role of L1 in promoting tumor cell adhesion and migration is discussed. The question of how L1 determination in tumor tissue samples, serum and ascites could potentially improve the diagnosis and monitoring of gynecologic tumor patients is also addressed. The presence of L1 in tissue and serum was found to be associated with recurrent disease and short survival, independently of the tumor's histological type. This provides an alternative classification of gynecologic tumors according to their aggressiveness rather than their histology. L1 expression was correlated with disease progression even in patients with Stage I endometrioid-type endometrial tumors, identifing them as high-risk patients on preoperative curettage specimens. Monitoring of soluble L1 during the follow-up period was found to signal disease progression and recurrence before clinical symptoms occur. L1-based diagnosis and prognosis has the potential to contribute to an improved disease management and could represent the basic rationale for novel tailored therapy.