Meningitis in the neurosurgical patient. In the patient with a basilar skull fracture and CSF leak, the risk of meningitis is greatly increased. The diagnosis of both leak and infection can be obscured by the patient's other injuries, and requires aggressive investigation of symptoms that suggest infection. Although the diagnosis is made with CSF cultures, when clinically suspected, treatment should begin after appropriate cultures have been obtained. Treatment should be directed against the most likely organisms, Streptococcus pneumoniae, Haemophilus influenzae, and the other organisms common to the upper respiratory tract. There are no good indications for prophylactic antibiotic usage in patients with known CSF leaks. The patient with a shunt or other CNS prosthetic device may have various manifestations of infection, depending on the type of device and its termination. Frank meningitis or ventriculitis is not always present. Diagnosis requires direct culturing of the shunt milieu, with the most frequent isolates being staphylococcal species and gram-negative enteric bacilli. The most effective therapy, for both eradication of the infection and minimization of the duration and morbidity of therapy, involves removal of the infected shunt, external drainage during parenteral antibiotic therapy, and complete replacement of hardware at the time of internalization. The postsurgical patient will not develop meningitis very frequently, but like the posttrauma patient, concurrent factors can make the diagnosis difficult. Differentiating infectious from chemical meningitis must often be initially based on CSF cell counts and chemistries alone. Treatment to cover the most likely organism, staphylococcal species and respiratory flora, should be started before the culture results are finalized. 