Central nervous system (CNS) infections in immunocompromised hosts are often accompanied by subtle disorders because immunosuppression usually decreases the inflammatory response. CNS infections in immunocompromised patients are usually caused by organisms different from those found in the general population. The organism causing CNS infection in an immunocompromised host can often be predicted if the type of immune abnormality of the patient is known. The common causes of CNS infection in immunocompromised hosts are reviewed here. Meningitis in patients with neutropenia is usually due to enteric Gram negative bacilli that live in the patient's own digestive tract. Pseudomonas aeruginosa is most common and is followed by E. Coli, Klebsiella, Enterobacter and Proteus. A major risk in patients with abnormal immunoglobulins or splenectomy is infection with encapsulated bacteria, particularly Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis. Meningitis caused by any of the encapsulated bacteria can be fulminant. Listeria monocytogenes is the most common cause of bacterial meningitis in patients with impaired cellular immunity. Nocardia asteroides is a leading cause of brain abscess in patients with hematologic malignancy. Most patients have evidence of concomitant pulmonary lesions. Fungi are among the most common organisms involving the CNS in immunocompromised hosts. Susceptible patients include those with lymphoma or leukemia and those who receive therapies aimed at suppressing delayed hypersensitivity. Cryptococcus neoformans is a common fungal cause of CNS infection in immunocompromised hosts. The primary site of infection is the lung. Spread to the CNS is via the blood stream. The clinical course is highly variable: meningitis, meningoencephalitis and focal mass lesions. Candida causes meningitis or meningoencephalitis characterized by multiple small abscesses in neutropenic hosts. Organisms reach the CNS via the blood stream usually from the digestive tract or infected intravenous catheters. Aspergillus causes brain abscess, cerebral infarction and focal meningitis in patients with neutropenia. The primary infection is in the lung. The parasites that infest the CNS of immunocompromised patients are usually those that exploit a T-lymphocyte, mononuclear phagocyte host defect. The most common are Toxoplasma gondii and Strongyloides stercoralis. There have been a few cases of amebiasis with dissemination to the brain in patients with hematologic malignancies. Toxoplasma gondii causes major CNS disease in immunocompromised hosts: meningoencephalitis or mass lesions.(ABSTRACT TRUNCATED AT 400 WORDS)