This study compares two sources of information on prior use of antibiotics in children with acute lower respiratory tract infection. The presence of antibiotics in respiratory specimens complicates recovery of bacterial pathogens and the selection of appropriate antibiotic treatment. The first source of information is the parents, who are asked about recent use of antibiotics by their child. The second source is an agar diffusion assay that detects antibiotics in urine specimens. In Argentina, where antibiotics are readily available without prescription, parental information about a child's recent antibiotic therapy was found to be relatively reliable only when their answer was affirmative.