Besides several side effects including anaphylaxis, blue dyes are also known to cause false pulse oximeter readings. We aimed to examine the effects of subareolar isosulfan blue injection on pulse oximeter (SpO2) readings. The study group included 27 patients undergoing SLNB using both radiocolloid and isosulfan blue. Another group of 27 patients constituted the control group. Pulse oximeter readings were compared. SpO2 decline a parts per thousand yen4 % was defined as significant. All but one (96.2 %) of the patients in the study group showed SpO2 declines, compared to only one patient in the control group. Median +/- Interqartile Range (IR) SpO2 decrease was 3.0 +/- 4.0 % in the study and 0.0 +/- 1.0 % in the control group (p < 0.001). There were significant (a parts per thousand yen4 %) SpO2 decreases in 13 (48.1 %) patients in the study group. Statistically significant differences were noted between the two groups in all recordings between 15 and 180 min (p < 0.001). Initial time for SpO2 fall and the time to the lowest SpO2 recording were 10.0 +/- 10.0 and 40.0 +/- 30.0 min respectively. Using subareolar injection, the frequency of false readings is comparable with intraparenchymal injections, and is higher than intradermal injections. Time to peak SpO2 fall, and the recovery period, are delayed in the subareolar technique.