Urinary albumin excretion (AE) was determined by a sensitive method (below dipstick positive values, 15 to 300 micrograms/minute) in 68 children with meningitis during 48 hours after hospital admission; 51 children had bacterial meningitis (BM) and 17 had aseptic meningitis. AE (results as mean +/- SD) during 0 to 24 hours was higher (P < 0.001) in patients with BM (36 +/- 40 micrograms/minute) than with aseptic meningitis (7 +/- 5 micrograms/minute), albeit no cutoff value distinguished the two conditions accurately. In BM the clinical course (uneventful, intermediate, complicated, fatal) correlated with AE of 0 to 24 hours (r = 0.34, P < 0.05) and AE of 25 to 48 hours (r = 0.63, P < 0.001). Cerebrospinal fluid protein concentration 24 to 36 hours after initiation of treatment correlated with AE of 25 to 48 hours (r = 0.34, P < 0.05). An index obtained by dividing AE by the weight of the child predicted the severity of clinical course more precisely (77% sensitivity, 85% specificity) than AE alone. Hence renal AE is an easily and non-invasively determined acute phase reactant of potential value as an early estimate of severity of BM.