Background: Data comparing survival after lobectomy versus that after segmentectomy for stage I nonsmall cell lung cancer (NSCLC) are limited to single-institution observational studies and 1 clinical trial. We sought to determine if lobectomy offers a survival advantage over segmentectomy for stage I NSCLC based on population-based data. Methods: Using the Surveillance Epidemiology and End Results (SEER) database (1998 to 2007), we identified patients who underwent either anatomic segmentectomy or lobectomy. Wedge resections were excluded. Analysis was limited to patients with stage I adenocarcinoma or squamous cell carcinoma. After stratifying patients based on tumor size (less than or equal to 2.0 cm, 2.1 to 3.0 cm, and 3.1 to 7.0 cm), we assessed for association between extent of resection and survival using the Kaplan-Meier method. To adjust for potential confounding variables, we used Cox proportional hazards regression models. Results: There were 14,473 patients who met our inclusion criteria. Lobectomy conferred superior unadjusted overall (p < 0.0001) and cancer-specific (p = 0.0053) 5-year survival compared with segmentectomy. Even after adjusting for patient factors, tumor characteristics, and geographic location, we noted that patients who underwent lobectomy had superior overall and cancer-specific survival rates, regardless of tumor size. Squamous cell histologic type, male sex, low lymph node counts, and increasing age, tumor size, and grade were all independent negative prognostic factors. Conclusions: Using a population-based data set, we found that lobectomy confers a significant survival advantage compared with segmentectomy. Our results provide additional evidence supporting the role of lobectomy as the standard of care for resection of stage I NSCLC regardless of tumor size.