Cerebrospinal fluid lymphocytosis (more than 50 percent lymphocytes or mononuclear cells) occurred in 14 of 103 cases of bacteriologically proved acute bacterial meningitis. Patients with cerebrospinal fluid lymphocytosis accounted for 32 percent (13 of 41) of all patients with bacterial meningitis with a cerebrospinal fluid white blood cell concentration of 1,000/mm3 or less. Cerebrospinal fluid lymphocytosis was significantly more common in neonates and in those without meningismus, but occurred in all ages and without any clear identifying clinical characteristics. The most common etiologic organisms were Streptococcus pneumonia (five), Neisseria meningitidis (two), and Hemophilus influenzae (two). Cerebrospinal fluid lymphocytosis is common in acute bacterial meningitis when the cerebrospinal fluid white blood cell concentration is below 1,000/mm3. It is therefore of little value in differentiating bacterial meningitis from viral, fungal, and tuberculous meningitis.