This study was conducted to confirm cefadroxil's antimicrobial efficacy and the in vitro indications that this drug may have an immunoglobulin E (IgE)-related immunomodulatory effect. Thirty-three patients ages 6 months to 12 years suffering from Staphylococcus aureus superinfected atopic dermatitis and having elevated serum IgE levels were randomized to receive either cefadroxil 50 mg/kg/day in two equal doses or placebo for 2 weeks. All patients treated with cefadroxil showed complete clinical resolution of the superinfection, compared with only 40% of the placebo group (P < 0.0001). Only one resistant isolate was identified. Although there were strong within-group improvements in the allergy-related signs and symptoms, no differences in atopic symptom improvement were seen between the treatment groups. The IgE levels did not change appreciably. These results suggest that, from an antibacterial viewpoint, cefadroxil is a useful adjunct in the treatment of superinfected atopic dermatitis when S aureus is involved.