Results: The median tumor size was 2.9 cm. Lymphatic, vascular, and pleural invasions were present in 39 (34.2 %), 50 (43.9 %), and 25 (21.9 %) patients, respectively. There were significant differences in recurrence-free and overall survival between patients with and without lymphatic invasion (P = 0.044 and P = 0.040, respectively). Multivariate Cox proportional hazards models demonstrated that postoperative complications (hazard ratio 3.37, 95 % confidence interval 1.53–7.42, P = 0.003) and lymphatic invasion (hazards ratio 2.76, 95 % confidence interval 1.26–6.04, P = 0.011) were independent prognostic factors influencing recurrence-free survival. Furthermore, age (hazard ratio 1.10, 95 % confidence interval 1.02–1.18, P = 0.013) and lymphatic invasion (hazard ratio 3.54, 95 % confidence interval 1.33–9.42, P = 0.011) were independent prognostic factors influencing overall survival.; Conclusions: Lymphatic invasion is an independent prognostic factor influencing both recurrence-free and overall survival in patients with pathological stage I squamous cell carcinoma of the lung. Patients with lymphatic invasion may be candidates for adjuvant chemotherapy.; Objective: The purpose of this study was to investigate the prognostic factors for pathological stage I squamous cell carcinoma of the lung to aid decisions regarding adjuvant chemotherapy.; Methods: We retrospectively analyzed data from 114 consecutive patients with completely resected pathological stage I squamous cell carcinoma of the lung by lobectomy or segmentectomy with systematic lymphadenectomy. © 2014, The Japanese Association for Thoracic Surgery.