Several molecular tests have been developed to estimate risk of distant recurrence and help clinical decision-making regarding adjuvant chemotherapy in patients with early stage breast carcinoma. Both Oncotype DX, a 21-gene expression profile, and Mammostrat, an immunohistochemistry-based assay, are validated to stratify patients into groups with low, intermediate and high risk of distant recurrence. However, they have not been compared head-to-head and little data are available regarding their correlation with clinicopathologic tumor features. In this study, we compared the clinicopathologic tumor features with risk estimations by Oncotype DX and Mammostrat in 106 low-grade estrogen receptor (ER)-positive breast carcinomas. Double immunohistochemical stain for pancytokeratin and Ki-67 was performed to assess cell proliferation in cancer vs stromal/inflammatory cells. Tumors showing intermediate/high risk by Oncotype DX, but not by Mammostrat, showed increased stromal cellularity, presence of inflammatory cells and increased proliferation in stromal/inflammatory cells. Discrepant cases showing intermediate/high risk by Oncotype DX but low risk by Mammostrat were associated with increased stromal cellularity, presence of inflammatory cells and increased proliferation in stromal/inflammatory cells, compared with concordant cases showing low risk by both assays. Our results suggest that low-grade ER-positive breast carcinomas with increased stromal/inflammatory cell proliferation may show an apparent increased risk of distant recurrence as assessed by Oncotype DX, which uses RNA extracted from a mixture of tumor and stromal/inflammatory cells in the assay. Mammostrat, which examines cancer cells only, may provide a better estimation of likely tumor behavior in a subgroup of low-grade breast carcinomas.