Parental history on antibiotic use and the urine antibacterial assay (UABA) result were compared in a study on Filipino children with acute lower respiratory tract infection (ALRI). Among 108 patients in whom urine for the UABA could be collected prior to starting antibiotic treatment in the hospital, 59 (55%) guardians reported preceding antibiotic use, 54% of whom were positive in the UABA. In another 37 (34%), the UABA result was positive, indicating nonreported use of antibiotics. Among 190 patients in whom urine could be collected only after intravenous administration of antibiotic, the UABA demonstrated large inhibition zones after the first dose in most patients but a negative result was seen in 14 cases. The inhibition zone radius was significantly smaller for chloramphenicol than for beta-lactam antibiotics (8.3 mm versus 16.1 mm after one dose; 95% confidence intervals = 7.0-9.7 and 14.9-17.2, respectively). Parental history on antibiotic use gives an underestimate of preceding antibiotic use in children with ALRI in the Philippines. The result partly explains the low yield of blood culture in many studies on ALRI, and stresses the need to develop new diagnostic methods not based on culture for those organisms highly sensitive to antibiotics such as Streptococcus pneumoniae and Haemophilus influenzae.