Arylamine N-acetyltransferase type 1 (NAT1) is reported to be involved in the transfer of an acetyl group from acetyl-CoA to the terminal nitrogen of hydrazine and arylamine drugs or carcinogens. Gene-specific hypomethylation frequently occurs in a range of cancers and hypomethylation of the genes often correlates well with increased transcription levels. This study was conducted in order to evaluate the methylation status and the transcriptional activity of NAT1 in breast cancer tissues (n=72), benign breast tissues (n=31) and morphologically normal breast tissues (n=30). Our findings showed that the methylation of the NAT1 gene was identified in 39 of the breast carcinomas (54.2%), 23 normal (76.7%) and 25 benign breast tissue samples (80.6%). The breast cancer tissues showed significantly lower methylation rates of the NAT1 promoters than the normal and benign tissues (P=0.012). Furthermore, cancer tissues showed lower methylation density rates than normal and benign breast tissues (P=0.012). The tissues that showed aberrant methylation of NAT1 showed significantly less mRNA expression compared with the unmethylated cases by a thousand fold (P<0.001). Twenty cancers from the methylated group showed positive staining for the estrogen receptor (ER) (51.3%), while 72.7% from the unmethylated group stained positive (P=0.063). Our results suggest that DNA hypomethylation in the NAT1 gene appears to be present in cancerous breast tissues thus indicating that this type of methylation may significantly influence the transcriptional activation of the gene. Therefore, hypomethylation of the NAT1 gene plays a significant role in breast carcinogenesis.