Surgical specimens from 111 patients with Stage I endometrial cancer were reviewed for the presence of lymph-vascular space invasion by tumor cells. Lymph-vascular space invasion was noted in 16 cases, and occurred most frequently in poorly differentiated tumors with deep myometrial penetration. Tumor recurrence developed in 44% of patients whose tumors demonstrated lymph-vascular space invasion as opposed to only 2% of patients without this finding (p less than 0.001). Of seven patients with lymph-vascular space invasion who experienced tumor recurrence, five developed extra-pelvic metastases. Discriminant function analysis of these data revealed a statistically significant correlation between lymph-vascular space invasion and tumor recurrence, independent of histologic differentiation of myometrial penetration. These findings suggest that lymph-vascular space invasion by tumor cells is an important prognostic variable in Stage I endometrial cancer which should be considered in treatment planning.