The significance of inflammation in carcinoma of the breast is controversial. Little attention has been paid to different patterns of inflammation or inflammation associated with different histological types of carcinoma. We have looked at the pattern of inflammation in 123 invasive mammary carcinomas (including 46 lobular), and characterised the inflammatory cells with immunohistochemistry in 21. We found different patterns of inflammation in ductal and lobular carcinoma. Diffuse inflammation was seen more in ductal carcinoma, particularly of high grade, and was predominantly composed of macrophages and T cells. It was associated with necrosis, but the correlation was weak, suggesting that other factors are important. Perilobular inflammation was seen most frequently in lobular and high-grade ductal carcinomas, particularly at the tumour edge. Perivascular inflammation was also largely at the tumour edge, but was not more common in any tumour type. In contrast to the diffuse inflammation, the perivascular and perilobular inflammation was composed of T and B cells. Normal lobules at the tumour edge showed consistent expression of HLA-DR, whereas lobules away from the tumour were negative. A combination of perilobular and perivascular inflammation composed of B and T cells with epithelial expression of HLA-DR mimicking lymphocytic lobulitis was seen more frequently in lobular than ductal carcinoma.