Aspergillus fumigatus is the most frequent strain in paranasal sinus infections associated with Aspergillus species. In this paper, a case of maxillary sinusitis caused by A. fumigatus with galactomannan, 1,3 beta D-glucan negativity is presented. The case was a 26-year old male diagnosed diagnosed to have biphenotypic leukemia and neutropenia and receiving chemotherapy. Maxillary sinusitis was detected in the paranasal sinus on computarized tomography taken upon presence of fever, cough, nasal pain, obstruction and bleeding. Galactomannan, 1,3 beta D-glucan antigenic test was found to be negative. Aspergillus fumigatus grew in the sample taken from the lesion in the left lower cochea of the nose. The results of in vitro antifungal susceptibility test with the isolate were found to be 0.25, 0.19, 0.032 and 2 μg/mL for amphotericin B, voriconazole, caspofungin and itraconazole, respectively. The patient recovered with amphotericin B treatment. Fungal sinusitis, particularly that caused by A. fumigatus should be considered in the differential diagnosis in sinusitis in immunocompromised patients even if their antigene tests are negative.