Abstract: The present invention involves a method for early detection of developing tamoxifen resistance in breast cancer. Tamoxifen is the drug of choice for hormonal therapy of a first recurrence of breast cancer, but its use is associated eventually with emergence of resistant tumors. Whereas initial treatment is usually followed by tumor regression, resistant tumors may actually resume growth under continued tamoxifen treatment. Because such growth may actually be augmented by the tamoxifen, it is essential to identify the onset of resistance as early as possible so alternative therapy may be promptly instituted. Monitoring increased levels of estrogenic tamoxifen isomers or estrogenic tamoxifen metabolites signals developing resistance.