A previously well 4-year-old boy presented to the emergency room with progressive cyanosis, pallor and vomiting over the last 5 h. Oxygen saturation on pulse oximetry was 87–89% despite 9 L/min of supplemental oxygen. He was tachypnoeic and had a systolic heart murmur, with no other findings on clinical examination. In his medical history, there was record of a restrictive atrial septal defect, with a normal echocardiogram from 3 years before. He had no relevant family history. His shoes appeared to have been recently painted, which raised the suspicion of methaemoglobinaemia, presumptively caused by aniline-containing shoe dye. The shoes were removed promptly and his feet washed profusely. After confirming the diagnosis, methylene blue was started. The level of methaemoglobin decreased rapidly and the boy made a full recovery.