Objectives: To identify the optimal cutoff value in Hounsfield units (HU) of maxillary sinus (MS) opacity and bone thickness (neo-osteogenesis) of MS as radiological predictors for mycetoma. Methods: One hundred and sixty-four patients, including 59 patients with unilateral MS mycetoma, 31 with unilateral odontogenic maxillary sinusitis, 44 with chronic rhinosinusitis and 30 with rhinitis, who underwent sinus or turbinate surgery were recruited. The bone thickness, HU of the MS posterolateral wall and sinus opacity were evaluated using computed tomography scan. Results: The bone thickness of the MS posterolateral wall in the mycetoma group was significantly higher than that in the odontogenic sinusitis and chronic rhinosinusitis (CRS) groups (p < 0.0001). The HU of the sinus opacity in the mycetoma group were significantly higher than those in the odontogenic and CRS groups (p < 0.0001). An optimal cutoff HU of sinus opacity >101.17 yielded a sensitivity of 96.6 and specificity of 100% for the diagnosis of MS mycetoma. An optimal cutoff of bone thickness >0.305 cm yielded a sensitivity of 84.7 and specificity of 60% for the diagnosis of MS mycetomas. Conclusions: The radiographic density measurement of MS opacification has a high predictive value for the diagnosis of MS mycetoma while radiographic neo-osteogenesis has not.