For women with nonmetastatic breast cancer, radiation therapy is recommended as a necessary component of the breast conserving surgery (BCS) treatment option. The degree to which Medicaid-enrolled women complete recommended radiation therapy protocols is not known. We evaluate radiation treatment completion rates for Medicaid enrollees aged 18-64 diagnosed with breast cancer. We determine clinical and socio-demographic factors associated with not starting treatment, and with interruptions or not completing radiation treatment. Using data from the Washington State Cancer Registry linked to Medicaid enrollment and claims records, we identified Medicaid enrollees diagnosed with breast cancer from 1997 to 2003 who received BCS. Among the 402 women who met inclusion criteria, 105 (26%) did not receive any radiation. Factors significantly associated with not receiving radiation included in situ disease and non-English as a primary language. Among those who received at least one radiation treatment, 65 (22%) failed to complete therapy and 71 (24%) patients had at least one 5 to 30 day gap in treatment. We found no significant predictors of interruptions in treatment or early discontinuation. A substantial proportion of Medicaid-insured women who are eligible for radiation therapy following BCS either fail to receive any treatment, experience significant interruptions during therapy, or do not complete a minimum course of treatment. More effort is needed to ensure this vulnerable population receives adequate radiation following BCS.