Background: Lymph/vascular space invasion (LVSI) is an important adverse prognostic factor in endometrial cancer (EC). However, its role in relation to type of recurrence and adjuvant treatment is not well defined, and there is significant interobserver variation. This study aimed to quantify LVSI and correlate this to risk and type of recurrence. Methods: In the post operative radiation therapy in endometrial carcinoma (PORTEC)-trials stage I EC patients were randomised to receive external beam radiotherapy (EBRT) versus no additional treatment after surgery (PORTEC-1, n = 714), or to EBRT versus vaginal brachytherapy (PORTEC-2, n = 427). In tumour samples of 926 (81.2%) patients with endometrioid tumours LVSI was quantified using 2-, 3- and 4-tiered scoring systems. Cox proportional hazard models were used for time-to-event analysis. Results: Any degree of LVSI was identified in 129 cases (13.9%). Substantial LVSI (n = 44, 4.8%) using the 3-tiered approach had the strongest impact on the risk of distant metastasis (hazard ratio (HR) 4.5 confidence interval (CI) 2.4-8.5). In multivariate analysis (including: age, depth of myometrial invasion, grade, treatment) substantial LVSI remained the strongest independent prognostic factor for pelvic regional recurrence (HR 6.2 CI 2.4-16), distant metastasis (HR 3.6 CI 1.9-6.8) and overall survival (HR 2.0 CI 1.3-3.1). Only EBRT (HR 0.3 CI 0.1-0.8) reduced the risk of pelvic regional recurrence. Conclusions: Substantial LVSI, in contrast to focal or no LVSI, was the strongest independent prognostic factor for pelvic regional recurrence, distant metastasis and overall survival. Therapeutic decisions should be based on the presence of substantial, not 'any' LVSI. Adjuvant EBRT and/or chemotherapy should be considered for stage I EC with substantial LVSI. (C) 2015 Elsevier Ltd. All rights reserved.