Purpose: Alteration in oxygen saturation is a side effect of the dye used in sentinel lymph node biopsy. The object of this study was to determine the possible effects of methylene blue on the oxygen saturation in patients undergoing sentinel lymph node biopsy. Methods: The complete peroperative records of 148 patients who underwent sentinel lymph node biopsy with intraparenchymal injection of methylene blue were reviewed. The data reviewed included the preinjection pulse oximeter saturation readings and the postinjection values until the readings returned to the preoperative levels in the postanesthesia care unit. Results: The pulse oximetry values were recorded preoperatively, after intubation and, at 15, 30, 45, and 60 minutes of the operation. The results showed that methylene blue did not cause any significant changes in oxygen saturation levels. Conclusion: We suggest that methylene blue might be preferable for the patients with concomitant disease, and for whom close monitoring of their oxygen saturation is required.