OBJECTIVES: Cancer is the second leading cause of death in the United States and a major contributor to health care expenditures. Our objective was to investigate whether disparities in outpatient treatment costs exist among an economically homogenous group - those on Medicaid. METHODS: Utilizing Maryland Medicaid administrative claims data, a retrospective cohort design was employed to examine disparities in ambulatory treatment costs of breast, colorectal and prostate cancer treatments by region, race and gender. We report mean and median results by each demographic category and test for the statistical significance of each. Lorenz curves are plotted and Gini coefficients calculated for each type of cancer. RESULTS: We do not find a consistent trend in ambulatory costs across the 3 cancers by traditional demographic variables. Lorenz curves indicate highly unequal distributions of costs. Gini coefficients are 0.687 for breast cancer, 0.757 for colorectal cancer, and 0.774 for prostate cancer. CONCLUSIONS: Significant variation in non-hospital based expenditures exists for breast, colorectal and prostate cancers in a population of homogeneous socio-economic status and uniform insurance entitlement. Observed individual level disparities are not consistent across cancers by region, race or gender, but the majority of this low income population receives very little ambulatory care.