Medullary carcinoma is famous of lymphoid infiltrate and improved prognosis. Cytotoxic T lymphocyte with positive Granzyme B constitutes a major subset within the infiltrate. The aim was to study Granzyme B expression and medullary carcinoma behavior. Twenty six female patients were admitted at Dr Fakeeh and Al Hayat Hospitals in Jeddah, K.S.A. from March 2004 to January 2007. Sixteen patients were below 45 years and ten were above. Granzyme B was positive in 75% of the first group and 60% in the second group. Granzyme B expression was positive in 62.5% of infiltrating ductal carcinoma with in situ component while it was 80% without in situ. Granzyme B expressed in 100% of medullary carcinoma, and 60% in others (p < 0.001). 4 lymph nodes out of six were affected in the medullary carcinoma while in others it was 18 out of 20 with no significant statistical differences. Granzyme B+ve Cytotoxic T lymphocyte were 100% in medullary carcinoma while Granzyme B+ve tumor cell was 33.3%. The values were 60% and 20% in others. N0 showed 100% +ve Granzyme B, N1 showed 71.4% and N2 showed 50%. Granzyme B, in a way or another is responsible for damaging cancer cells, leading to lost ability to invade lymphatic vessels or to grow in regional lymph nodes.