Abstract An open, randomized clinical trial was performed to evaluate the effects of cefuroxime axetil, compared with those of cefaclor, on the fecal flora of children with acute infectious diseases. Thirty-two children were randomized to receive either cefuroxime axetil or cefaclor. Fecal samples were collected 24 hours before treatment, 3 and 6 to 7 days after the start of treatment, and 7 days after the completion of treatment. Cefuroxime axetil induced few changes in aerobic fecal flora, causing only a slight increase in Aerococcus viridans and a slight decrease in Enterobacter cloacae ( P < 0.05), and no changes in anaerobic flora. Cefaclor induced more marked changes in bacterial flora; it decreased the aerobes lactobacilli ( P < 0.05) and Escherichia coli ( P < 0.001) and increased Citrobacter freundii ( P < 0.001) and Proteus mirabilis ( P < 0.01), while its effects on anaerobic microorganisms produced a reduction in Fusobacterium varium and Bacteroides ureolyticus ( P < 0.05). Clostridium difficile, absent during cefuroxime treatment, was observed in three children during or soon after cefaclor therapy. We conclude that cefuroxime axetil does not induce any clinically significant changes in the intestinal microflora.