Flutamide, an anti-androgen used in prostate cancer therapy, is also a derivative of aniline. Mild, usually asymptomatic, methaemoglobinaemia has been reported. We report a patient receiving flutamide therapy who developed cyanosis, dyspnoea and anaemia, initially attributed to marked methaemoglobinaemia by the CO-Oximeter method. An unsuccessful trial of methylene blue therapy led to the finding of marked sulphaemoglobinaemia. Sulphaemoglobinaemia has not previously been reported with flutamide use. Recognition of this association is important, given the refractoriness of sulphaemoglobinaemia to methylene blue therapy.