A 3-year prospective study is reported of the value of conventional cerebrospinal fluid (CSF) investigations in the diagnosis of meningitis. Of 739 cases of aseptic meningitis 74 (10 per cent) had a total CSF cell count > 500 .times. 106/l, a + or more of turbidity present on Pandy''s test, a CSF total protein > 0.8 Gl, or a CSF glucose < 2.2 mmol/l and would have been inappropriately treated for bacterial meningitis using these criteria alone as would 18 (17 per cent) of 108 confirmed viral meningitis cases. Conversely, 10 (7 per cent) of 140 .times. meningitidis cases, 3 (6 per cent) of 47 Haemophilus influenzae cases, and none of 34 S. pneumoniae cases had a total CSF cell count < 500 .times. 106/l, globulin absent or a trace on Pandy''s test, a CSF total protein < 0.8 G/l, a CSF glucose > 2.2 mmol/l and no organisms visible on microscopy. In a tuberculosis meningitis group of 62 children, 55 (89 per cent) had either a + or more of turbidity on Pandy''s test, a CSF protein > 0.8 G/l, a CSF glucose < 2.2 mmol/l or acid fast bacilli visible on microscopy. Pandy''s test is a simple, but valuable aid to the selection of cases requiring the institution of antibiotic therapy.