Both hereditary and sporadic ovarian tumors frequently have decreased BRCA1 expression. One mechanism of downregulating BRCA1 expression is hypermethylation of the BRCA1 promoter. Studies have shown that the BRCA1 promoter is aberrantly hypermethylated in a subset of ovarian tumors, although the proportion varies widely between reports. High-resolution analysis of the BRCA1 promoter in ovarian cancer may provide information regarding the extent and heterogeneity of methylation and guide future studies using methylation-specific polymerase chain reaction (MS-PCR). We screened 50 primary epithelial ovarian tumors for BRCA1 promoter hypermethylation using MS-PCR. The BRCA1 promoter was hypermethylated in 16% (8 of 50) of the tumors, including two stage IA tumors. Sequence analysis of the promoter revealed that methylation of the CpG island is both extensive and mosaic in the methylated samples. Two CpG dinucleotides in the BRCA1 promoter, within and adjacent to a Myb consensus binding site, were most frequently methylated in ovarian tumors. BRCA1 expression was significantly lower in methylated than in unmethylated samples. Our analysis of the BRCA1 promoter revealed preferential methylation of specific CpG sites in ovarian tumors. This finding could be exploited in the design of highly sensitive MS-PCR assays for direct assessment of tumor DNA and potentially for early detection of ovarian cancer in body fluids.