Sentinel lymph node (SLN) biopsy is an accurate technique to determine the metastatic status of lymph nodes. Radionuclide guidance makes the procedure easier and improves the success rate. Coordinating the operating room and nuclear medicine schedules causes delays when same-day radionuclide injection is used. We hypothesized that injection of 99m-Tc sulfur colloid immediately preoperatively is effective in SLN detection. We analyzed a prospective database of 70 patients treated at Harbor-UCLA Medical Center. The first 39 patients underwent completion axillary dissection (Group A) and subdermal injection of sulfur colloid immediately before surgery. A second group of 31 patients (Group B) had intraparenchymal injection immediately before surgery. We used isosulfan blue in all cases. SLNs were identified in 97 per cent of cases. SLNs were radioactive in 94 per cent and blue in 90 per cent. In Group A the accuracy and the positive and negative predictive values of SLN biopsy were 100 per cent. SLN counts per second ranged from 22 to 1700. The mean count per second was 290 +/- 281 (mean +/- standard deviation). Subdermal and intraparenchymal injections were equally successful (93% vs 92%). In conclusion injection of radiocolloid and isosulfan blue immediately preoperatively is highly successful and accurate in the detection of SLNs in breast cancer. It avoids operating room schedule delays.