Expression of the inducible isoform of the cyclooxygenase gene (PGHS-2, COX-2) which codes for the enzyme that catalyzes formation of prostaglandins, was detected in 13/13 human breast tumors of high grade but not in samples of normal breast tissue. There was a statistically significant linear association between COX-2 gene expression and high (>50%) tumor cell density (p<0.01), with COX-2 protein localized to tumor cells. These results indicate that COX-2 gene expression may be useful as a molecular biomarker for human breast tumors and may also predict sensitivity to treatment with nonsteroidal anti-inflammatory drugs.