An immunohistochemical survey of X-linked inhibitor of apoptosis (XIAP) expression in mammary carcinoma was performed. XIAP, the most potent of the inhibitor of apoptosis family of caspase inhibitors, has been linked to tumor aggressiveness and therapeutic resistance in several malignancies and is considered an attractive target for cancer drug discovery. Routinely processed sections from 94 ductal carcinomas, 9 lobular carcinomas, and 10 ductal carcinomas with lobular components or features were subjected to citrate-based antigen retrieval, immunostained with anti-XIAP (BD Biosciences, Franklin Lakes, NJ), Envision+ reagents (Dako, Carpinteria, CA), and diaminobenzidine. Positive staining was found in 22.7% of grade 1, 44% of grade 2, and 89.5% of grade 3 ductal carcinomas. Strong staining occurred in no cases of grade 1, 13% of grade 2, and 55.2% of grade 3 ductal carcinomas. XIAP staining increased overall with grade of ductal carcinoma in situ as well. The staining intensity of invasive carcinoma correlated with that of the corresponding ductal carcinoma in situ in 70% of cases. Ductal carcinomas overall showed more staining than lobular carcinomas. XIAP is most strongly and commonly detected in grade 3 ductal carcinoma. The degree of XIAP expression appears frequently to be determined in the preinvasive intraductal phase of tumorigenesis. These findings suggest a possible role of XIAP in the more aggressive clinical behavior of grade 3, compared with lower-grade ductal carcinomas.