A sinonasal infection is a frequent complication in patients with haematological malignancies, and may represent a challenge in terms of differential diagnosis between a bacterial or fungal infective process and tumour localization. A timely and correct diagnosis in these patients is critical and, therefore, may require consultation of specialists outside of haematology; an incorrect diagnosis which underestimates the seriousness of the infection can be fatal. Symptomatic trigeminal neuralgia resulting from direct compression or perineural invasion from malignancy is not uncommon in the literature. However, trigeminal neuralgia as an isolated symptom at the onset of a bacterial or invasive fungal sinusitis is rare and risks going unnoticed. The authors herein describe three cases of patients affected by acute myeloid leukaemia or lymphoma in which an invasive fungal sinusitis appeared at the onset as an isolated trigeminal neuralgia, with pain located along the distribution area of the second branch of the trigeminal nerve. Only after referring these patients to a neurologist for a host of neurological exams it was possible to confirm a diagnosis of secondary maxillary sinus fungal involvement.