OBJECTIVE We report a rare case of concurrent myeloid sarcoma and acute fulminant invasive fungal sinusitis in a patient with relapsed acute myeloid leukaemia. CASE REPORT A 73-year-old man was diagnosed with acute myeloid leukaemia and developed relapse one year later. After two courses of azacytidine, he began suffering from a dull pain in the left temporal and orbital regions. Sinus computed tomography showed a localised lesion in the left ethmoid sinus, which rapidly progressed to an extensive intracranial mass within one month. Surgical debridement was performed, and histopathological analysis revealed the coexistence of myeloid sarcoma and acute fulminant invasive fungal sinusitis. The patient responded well to prompt surgical debridement, antifungal medication and radiotherapy. CONCLUSION Coexistence of sinonasal myeloid sarcoma and acute fulminant invasive fungal sinusitis poses an urgent diagnostic and management challenge to clinicians. Timely recognition of this rare comorbid condition is warranted as application of appropriate treatment can save lives.