We evaluated the contribution of Tc-99m methylene diphosphonate (MDP) scintimammography (MDP-SMM) to the diagnosis of breast cancel in 96 patients with 109 (52 palpable, 57 nonpalpable) breast lesions and compared them with mammography. Mammographic classification of the lesions were done as probably malignant (29), indeterminate (29) and probably benign (51). There were 23 malignant and 86 benign lesions. MDP-SMM successfully detected 16 of 23 (70%) breast cancers. Twelve of the 13 palpable and 4 of the 10 nonpalpable cancers showed Tc-99m MDP accumulation. Eight of the 86 (9%) benign breast lesions showed Tc-99m MDP accumulation (false positives). In the mammographically indeterminate group, MDP-SMM changed 22 of the 24 false positives into true negatives and showed 2 false negatives. In the 35 lesions detected in dense breasts, 10 of the 13 false positives were changed into true negatives and showed 1 false negative. As a conclusion, combined use of MDP-SMM with mammography seems to be usefull in cases with mammographically indeterminate breast lesions and dense breasts and may help to preclude unnecessary breast biopsies.