Planar scintimammography with 99Tcm-sestamibi (99Tcm-MIBI) has been shown to be useful in diagnosing breast carcinoma. The aim of this study was to compare single photon emission tomography (SPET) and planar imaging for scintimammography with 99Tcm-MIBI in the detection of primary breast cancer and axillary lymph node involvement. Sixty-three females with mammographically suspicious lesions and 12 controls were evaluated. Dynamic images were acquired commencing immediately after the injection of the radiopharmaceutical, followed by multiple planar images in the supine and prone positions plus SPET supine imaging. A final histopathological diagnosis was achieved after surgery. A total of 66 breast lesions were considered. No focal uptake of 99Tcm-MIBI was observed in the breasts or axillas of the controls. In the patients with breast cancer, the sensitivity was 92.9% (39/42) for SPET, 71.4% (30/42) for supine and 85.7% (36/42) for prone planar imaging, respectively; the specificity was 87.5% for SPET and 91.6% for the planar scans. Metastatic axillary lymph node involvement was seen in 19 patients: the sensitivity was 84.2% (16/19) for SPET and 63.2% (12/19) for planar images; the specificity was 91.3% and 95.7% respectively. Our results confirm the high diagnostic accuracy of 99Tcm scintimammography in the diagnosis of breast cancer, and suggest that SPET is more sensitive than planar images, especially in detecting axillary lymph node involvement.