Comorbidity burden has been suggested as influencing early-stage breast cancer therapy but previous studies have not considered the severity of these comorbidities. Therefore, we examined the influence of comorbidity severity by age and race/ethnicity on early-stage breast cancer treatment over time. We used linked Surveillance, Epidemiology, and End Results (SEER)-Medicare data to determine whether comorbidity severity influences receipt of definitive and preferred early-stage breast cancer treatment and explains racial/ethnic and age disparities in receiving such therapy. Definitive surgical therapy was defined as any primary surgery other than breast conserving surgery (BCS) without radiation therapy (RT). Preferred surgical therapy was defined as BCS plus RT. Comorbidities were defined as either "unstable" (life threatening or difficult to control) or "stable" (less serious but with potential to influence daily activity). Surgical treatment trends from 1993 to 2005 were analyzed in regression models adjusting for comorbidity burden, age, and race/ethnicity in 93,596 elderly female Medicare beneficiaries with stage 1-2 invasive breast cancer. Receipt of BCS alone (compared with any definitive surgical therapy) was independently associated with neighborhood socioeconomic status, unmarried status (OR [odds ratio] 1.18, 95% CI: 1.12-1.23), tumor size (OR 0.78, 95% CI: 0.69-0.87 for tumors ≥4 cm vs. <2 cm), tumor grade (OR = 0.89, 0.88, and 0.81 for grades 2-4 vs. 1, respectively), stable comorbidities (OR = 0.76, 0.71, and 0.72 for 1, 2, and 3 vs. 0 stable comorbidities, respectively), and unstable comorbidities (OR 1.20, 95% CI: 1.14-1.28). Black women were 4-5% more likely to receive suboptimal therapy (BCS alone), even after adjusting for all available patient, tumor, and regional characteristics. Black race/ethnicity was associated with higher probability of receiving suboptimal treatment, independent of comorbidities, although we do not know whether this effect was due to clinicians' failure to offer RT or patients' failure to accept it.