The aim of this study was to determine the outcome of lumbar punctures (LPs) in critically ill medical patients and how likely the results were to change case management. A retrospective review was conducted on the medical records of all 168 patients who underwent LP during their medical intensive care unit (MICU) admission at a university hospital during a 4.5-year period beginning in January 2000. Lumbar puncture was performed a mean of 2.8 days after MICU admission. The most common symptoms that prompted LP were changes in mental status and fever. Seventy-four percent of patients were on antibiotics at the time of LP, and 98% of patients had a computed tomography scan of the head performed before the procedure. Lumbar puncture confirmed meningitis in 47 (30%) patients and provided a specific bacteriologic diagnosis in 5 (3%) patients. The results of the procedure led to a change in management in 50 (30%) patients. The presence of meningeal signs and use of antibiotics at the time of the procedure were the factors that predicted change in management. Although the likelihood that LP will yield a specific bacteriologic diagnosis in critically ill patients is low, the procedure frequently provides important information that can lead to a change in case management, most commonly de-escalation of antibiotic therapy.