BACKGROUND: Definitive surgical and radiation therapy (RT) treatments are evolving rapidly for stage I non-small cell lung cancer (NSCLC). We hypothesized that utilization of definitive therapies increased between 2000 and 2010 and that survival improved for stage I NSCLC patients over the same time period. Secondary objectives were determining trends in patterns of care and predictors of utilization. METHODS: Population-based, observational, comparative effectiveness study used Surveillance, Epidemiology, and End Results-18 data from 2000 to 2010. The main outcome measure was 2-year risk of death for stage I NSCLC. RESULTS: Between 2000 and 2010, 40,589 patients (62%) underwent surgery, 10,048 (15%) received RT, 2,130 (3%) received both surgery and RT, and 11,537 (18%) received neither surgery nor RT. Annually, the odds of receiving either definitive RT or undergoing surgery increased relative to the odds of receiving no treatment (odds ratio [OR] radiation 1.04, 95% confidence interval [CI]: 1.03 to 1.05; OR surgery 1.05, 95% CI: 1.04 to 1.05). Among surgical patients, the proportion of sublobar resections steadily increased from 12.9% to 17.9%. For all patients, the 2-year risk of death decreased by 3.5% each year (hazard ratio [HR] 0.965, 95% CI: 0.962 to 0.969), driven primarily by improved survival for surgical (annualized HR 0.959, 95% CI: 0.954 to 0.964) and RT (annualized HR 0.942, 95% CI: 0.935 to 0.949) patients. CONCLUSIONS: Between 2000 and 2010, stage I NSCLC patients were more likely to receive definitive treatment with either surgery or RT, leading to a decline in the number of untreated patients. Survival also improved substantially for stage I NSCLC patients, with the largest survival improvements observed in patients undergoing definitive RT.