The purpose of this study was to evaluate the feasibility of surgery radioguided with intravenous 99mTc sestamibi associated with magnetic resonance imaging (MRI) of the breast to detect tumor extent and guide complete tumor resection. A descriptive experimental study was developed with 10 breast cancer patients (stage IIA–IIB) who underwent mastectomy. From 2 to 10 days before surgery, the patients underwent breast MRI with 0.1 mmol/kg of gadolinium and scintimammography with a 740 MBq of 99mTc sestamibi. A region of interest was drawn around the tumor image and an uptake curve as a function of time was plotted to calculate the optimal time to perform radioguided surgery. In the perioperative period, the same dose of 99mTc sestamibi was intravenously injected into the patients. Tumor resection was performed under the guidance of a gamma probe. MRI was used to evaluate the skin and deep fascia involvement and to detect occult tumor foci which were also excised. Subsequently a modified radical mastectomy was performed. Tumor and residual breast were histopathologically examined. In a series of 10 women, all demonstrated 99mTc sestamibi uptake in tumor cells. Eight patients showed no disease in the residual breast, one presented with one foci of invasive ductal carcinoma measuring 0.5 cm in diameter located 5 cm from the tumor bed, and one presented with one foci of ductal carcinoma in situ measuring 0.8 cm at the resection margin. The mean tumor size in the histopathologic assessment was 3.3 cm and in MRI was 5.0 cm. Radioguided surgery using 99mTc sestamibi associated with MRI is a feasible technique that can be employed in tumor resection.