This report and the accompanying review of the literature address the challenges, when using surveillance magnetic resonance (MR) imaging, of establishing the origin of newly detected central nervous system lesions. Routine surveillance MR imaging in a 16-year-old boy, whose medulloblastoma had been successfully treated, demonstrated asymptomatic nodular leptomeningeal enhancement of the brain and spinal cord, which was consistent with recurrent disease. Examination of the cerebrospinal fluid, however, led to the diagnosis of bacterial meningitis. Two weeks after completion of antibiotic therapy, the original MR imaging findings were seen to have resolved. This case illustrates the importance of considering clinical and laboratory data, including results from a complete examination of the cerebrospinal fluid, when interpreting the origin of new lesions revealed by MR imaging.