The most important prognostic factor in breast (B) cancer (C) is axillary (A) lymph (L) node (N) status, and virtually all patients with BC undergo ALN dissection to assess N involvement. The aim of this study was to evaluate the accuracy of planar and tomographic Tc-99m tetrofosmin scintigraphy in the detection of ALN involvement in BC. A group of 85 female patients (age range: 31-82 years) with 87 BCs were studied before surgery. Three planar images, right and left prone lateral and supine anterior thoracic views, and 360 degrees supine thoracic single photon emission tomography (SPET) were acquired after Tc-99m tetrofosmin injection (740 MBq i.v.). ALN status was evaluated by histological exam after A dissection: metastatic ALN involvement was proved in 31 out of 87 cases. Sensitivity was 87.1% (27/31) for SPET and 61.3% (19/31) for planar images (p<0.01); specificity was 92.9% (52/56) and 94.6% (53/56), respectively, with a global accuracy of 90.8% (79/87) for SPET and 82.8% (72/87) for planar imaging (p<0.05). Sensitivity rose from 75% in non-palpable Ns to 94.7% in palpable ones for SPET, and from 41.7 to 73.7% for planar scans. SPET was positive in 17/18 (94.4%) patients with >3 metastatic Ns and in 10/13 (76.9%) with < or = 3 involved Ns, whereas planar images were positive in 14/18 (77.8%) and in 5/13 (38.5%) cases, respectively. In conclusion, our findings indicate that Tc-99m tetrofosmin scintigraphy is useful in the presurgical detection of ALN metastases in BC, with SPET more accurate than conventional planar images, thus suggesting its more frequent use in scintimammography; moreover, the total number of histologically involved Ns can affect the scintigraphic results.