Background: Early initiation of treatment following a cancer diagnosis is associated with better patient outcomes. Therefore, we compared time to treatment initiation in American Indian/Alaska Native (AIAN) vs. non-Hispanic White (NHW) cancer patients, with the goal of identifying a modifiable factor to improve survival among AIAN cancer patients. Methods: We used Surveillance, Epidemiology, and End Results (SEER) registry data linked to Medicare claims to identify new cases of colorectal, lung, or female breast cancer who were: 1) diagnosed from 2001-2007 with local or regional stage disease, 2) residing in a SEER catchment area, 3) enrolled in Medicare A and B at diagnosis, 4) ≥65 years old, and 5) AIAN or NHW. Cancer diagnosis date was obtained from SEER; the date of first surgical, chemotherapeutic, or radiation treatment was ascertained from Medicare claims. We used Cox regression models to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) comparing the rate of treatment initiation during the 6 months following diagnosis between AIAN and NHW patients, stratified by cancer site and stage. Model covariates were age, sex, diagnosis year, marital status, rural residence, and pre-diagnosis Klabunde comorbidity score. Results: 146,877 NHW and 522 AIAN patients were diagnosed with local or regional breast, colorectal, or lung cancer over the study period. Compared to NHWs, AIANs were younger (mean age 75.0 vs. 76.2y), more likely to live in rural areas (37% vs. 22%), and had higher co-morbidity scores (mean Klabunde 0.43 vs. 0.36). The rate of treatment initiation during the 6 months following diagnosis was lower in AIAN compared to NHW patients diagnosed with breast (HR=0.85, CI: 0.72-0.99), colorectal (HR=0.74, CI: 0.63-0.87), or lung (HR=0.82, CI: 0.67-1.00) cancer. Conclusions: Our results suggest that despite equal Medicare insurance coverage, AIANs may experience delays in treatment initiation compared to otherwise similar NHWs. Decreasing the time to treatment initiation may improve survival rates for AIAN cancer patients, who currently have among the lowest five-year survival rates of any racial group.