Background: Randomized trials have shown no survival difference for patients with stage I breast cancer treated with mastectomy versus breast-conserving surgery (BCS) with radiotherapy (RT). RT is recommended after BCS in order to decrease local recurrence and mortality. We sought to evaluate the treatment trends in patients with stage I breast cancer. Methods: We used the Surveillance, Epidemiology, and End Results (SEER) database to identify 194,860 women with stage I breast cancer diagnosed from 1988 to 2007. We evaluated factors that were associated surgical treatment and the utilization of RT after BCS. Results: There was a progressive decline in the proportion of patients with stage I breast cancer who were treated with mastectomy from 1998 to 2007. Significant predictors for being treated with mastectomy included single/divorced women (p = 0.007), white race (p < 0.001), estrogen receptor negativity (p < 0.001), earlier year of diagnosis (p < 0.001), smaller tumor size (p < 0.001), and region (p < 0.001). Twenty percent of the BCS cohort did not receive RT, and this proportion did not change over time. Significant predictors for not receiving RT included small tumor size (p < 0.001), African American race (p < 0.001), increasing age (p < 0.001), single/divorced women (p < 0.001), estrogen receptor negativity (p < 0.001), and region (p < 0.001). The survival for patients treated with BCS and RT was significantly higher than for those who did not receive RT (p < 0.001). Conclusions: The use of BCS for the treatment of stage I breast cancer increased over time. A constant proportion of patients did not receive RT after BCS. Omission of RT in BCS is associated with an increase in mortality.