Intracranial complications arising from frontal sinusitis occur infrequently. However, they can progress with such rapidity that the clinical situation becomes far advanced before they are recognized. Radiographic imaging techniques may not be definitive early in the course of these complications. The infectious disease service was asked to evaluate a middle-aged man with acute global headache and nasal discharge for two weeks. CSF pleocytosis (3,600 WBC/mm3) was documented on lumbar puncture, and a dense mass was noted on sinus radiographs. At surgery, a large bony lesion was found extending from the right frontal sinus into the adjacent ethmoid sinus and nasal-frontal duct. The authors discuss the bacteriology, pathogenesis, and potentially serious intracranial and extracranial complications of frontal sinusitis which were considered during their evaluation of this patient.