To achieve a more specific method to estimate the real size of breast cancer, we have developed a method to fuse magnetic resonance imaging (MRI) and scintimammography (SM) images. The aim of this study was to assess the feasibility of this method and to evaluate its accuracy to measure the size of breast cancer compared with MRI alone, mammography, and clinical examination, employing pathologic size as the gold standard. Twenty consecutive breast cancer women at stages IIA-IIIA, scheduled for mastectomies, underwent SM with (99m)Tc-sestamibi and MRI with gadolinium 2-10 days before surgery. All patients had had recent mammographies and were examined clinically. Software was developed in visual language to perform the fusion between MRI and SM images and tumor measurements (MRI/SM). The tumor size, in 3 diameters (anteroposterior, longitudinal, and transverse), for each examination was correlated with pathological measurements using linear regression. The MRI/SM technique was successfully performed in all patients, and the principal tumor was measured by this method. The MRI/SM cancer measurements correlated better with pathology than MRI, mammography, and clinical exam in all diameters analyzed (r = 0.88, 0.81, 0.81; SE = 0.11, 0.14, 0.11 in anteroposterior, longitudinal, and transverse diameters, respectively). The MRI/SM is a feasible technique and appears to be more accurate than other examinations (MRI alone, mammography, and clinical exam) to measure breast cancer size.142903-10