Mucosa associated lymphoid tissue (MALT) lymphomas arising in the breast are uncommon and few cases have been assessed for MALT lymphoma-associated translocations, BCL-10 expression, or NF-kappaB activation. In this study, we analyzed eight cases of primary breast MALT lymphoma. We also included 14 cases of primary breast diffuse large B-cell lymphoma since some of these may represent transformation of MALT lymphoma, known to occur at extra-mammary MALT sites. All cases were assessed for MALT1 gene rearrangements by fluorescence in situ hybridization (FISH). Using immunohistochemical methods, all cases were assessed for BCL-10, and subsets were assessed for NF-kappaB p65 and p50. None of the cases had MALT1 gene rearrangements by FISH. Of eight MALT lymphomas, BCL-10 was positive in seven (88%), with moderate nuclear and cytoplasmic staining in six, and a weak cytoplasmic staining in one. NF-kappaB p65 (n=8) and p50 (n=5) were negative or showed only cytoplasmic staining (ie inactivated) in all cases. Of 14 diffuse large B-cell lymphoma cases, BCL-10 was positive in 12 (87%), with weak-to-moderate cytoplasmic staining in 10, weak cytoplasmic and focally nuclear staining in one, and a moderate-to-strong nuclear and cytoplasmic staining in one. NF-kappaB p65 (n=11) showed cytoplasmic staining in all cases, whereas p50 (n=8) showed nuclear positivity (ie activated) in two (25%) cases. We conclude that MALT1 gene rearrangements are absent or rare in primary breast MALT lymphoma and diffuse large B-cell lymphoma. In MALT lymphomas, the moderate BCL-10 nuclear expression in six neoplasms is inconsistent with the FISH results, suggesting that BCL-10 immunostaining overestimates the frequency of MALT1 gene rearrangements. We also could not demonstrate NF-kappaB activation using nuclear staining for p65 and p50. In contrast, breast diffuse large B-cell lymphomas are heterogeneous. Weak cytoplasmic BCL-10 staining in most cases and evidence of NF-kappaB p50 activation in a subset differs from breast MALT lymphomas.