New biological knowledge and new diagnostic tools for breast cancer diagnosis may have an important impact on the strategies followed by oncologists in breast cancer management. So far, it has not been possible to label a nucleotide or its precursor with technetium-99m (99mTc) for use in in vivo studies. In this paper, we report the labelling of a precursor, the nitrogenate base thymine, with 99mTc. It has been used to evaluate breast lesions. After animal studies and with the consent of patients, it was employed for evaluation in humans. Thirty-nine women were studied: 32 patients and seven healthy volunteers. Twenty-three malignant lesions showed uptake of labelled thymine. Of the 12 benign cases, there was one false-positive uptake. Labelled thymine was also taken up by one case of malignant microcalcifications. Labelled thymine can be used as a diagnostic method in the identification of malignant anatomo-radiologic palpable lesions of the breast, observed by mammography, possibly avoiding unnecessary biopsies.