A 9 month-old girl was given subcutaneous isosulfan blue to outline lymphatic channels during surgery for thoracic duct ligation. Her pulse oximetry values rapidly declined to a nadir of 85%, 35 min after dye injection. Arterial blood gases revealed methemoglobinemia ([MetHb] = 6.5%). Although abnormal pulse oximetry has already been reported in association with isosulfan blue, methemoglobinemia has not previously been reported. The absorption spectrum for isosulfan blue was determined and when superimposed on that of methemoglobin it was found to have an overlying peak. Interference by the dye was postulated to have caused the abnormal methemoglobin result. The phenomenon was simulated in vitro by adding isosulfan blue to whole blood, and analysing it in the same blood gas analyser as was used for the case, as well as another for comparison. One blood gas analyser reported elevated methemoglobin concentration and the other did not. The samples were sent to a reference laboratory using a chemical method to detect methemoglobin to confirm that the elevated methemoglobin level was spurious.