Background: Evidence guiding adjuvant chemotherapy (AC) use after lobectomy for stage I non–small cell lung cancer (NSCLC) is limited. This study evaluated the impact of AC use and tumor size on outcomes using a large, nationwide cancer database. Methods: The effect of AC on long-term survival among patients who underwent lobectomy for margin-negative pathologic T1–2N0M0 NSCLC in the National Cancer Data Base from 2003 to 2006 was estimated using the Kaplan-Meier method. The specific tumor size threshold at which AC began providing benefit was estimated with multivariable Cox proportional hazards modeling. Results: Overall 3,496 of 34,360 patients (10.2%) who met inclusion criteria were treated with AC, although AC use increased over time from 2003, when only 2.7% of patients with tumors less than 4 cm and 6.2% of patients with tumors of 4 cm or larger received AC. In unadjusted survival analysis, AC was associated with a significant 5-year survival benefit for patients with tumors less than 4 cm (74.3% vs 66.9%; P<.0001) and 4 cm or greater (64.8% vs 49.8%; P<.0001). In subanalyses of patients grouped by strata of 0.5-cm increments in tumor size, AC was associated with a survival advantage for tumor sizes ranging from 3.0 to 8.5 cm. Conclusions: Use of AC among patients with stage I NSCLC has increased over time but remains uncommon. The results of this study support current treatment guidelines that recommend AC use after lobectomy for stage I NSCLC tumors larger than 4 cm. These results also suggest that AC use is associated with superior survival for patients with tumors ranging from 3.0 to 8.5 cm in diameter.