The utility of Medicaid claims for studying cancer care is not known. Our objective was to evaluate how well Medicaid claims capture diagnostic and treatment information recorded by the California Cancer Registry (CCR). We compared cancer treatment from Medicaid claims with CCR data, using 1988-2000 cases matched with 1997-1998 Medicaid enrollment data. Medicaid claims corroborated diagnoses for 73 percent of breast and 68 percent of colorectal cancers in CCR. Medicaid claims confirmed surgery for 67 percent of CCR's breast cancers. We found that Medicaid claims have moderate sensitivity for identifying cancer diagnoses and surgery. Linked registry-Medicaid data can identify indigent patients and the timing of Medicaid coverage.