Brain abscess is most commonly due to direct extension of infection from infected middle ear, mastoid sinus, or paranasal sinuses; traumatic injuries, or infected prostheses. There can also be more distant spread from the lungs or from infected emboli. There is increased incidence in immunosuppressed patients. The most frequent organisms cultured are various streptococci, Bacteroides, gram-negative organisms, and Staphylococcus aureus. Mixed infections are present in 30%-60% of cases. Apparently the CSF findings in brain abscess are not significantly influenced by the causative organism or the location of the lesion. About 10% of patients are said to have normal CSF test results. The remainder usually have a picture compatible with aseptic meningitis. The spinal fluid is most often clear, and about 70% of patients are said to have increased pressure. Protein levels are normal in nearly 25% of patients, in about 55% the values are between 45 and 100 mg/100 ml (0.45-1.0 g/L), and in the remaining 20% the values are more than 100 mg/100 ml. The CSF glucose level is normal. Cell counts are variable; about 30% are between 5 and 25, about 25% are between 25 and 100, and about 25% are between 100 and 500/cu mm. Lymphocytes generally predominate, but a significant percentage (5%-25%) of polymorphonuclear neutrophils are said to be nearly always present. In occasional cases, an abscess breaks through to the subarachnoid space and results in purulent meningitis. CT or MRI is very helpful in demonstrating intracerebral abscesses. Radionuclide brain scans are useful if CT is not available.