BACKGROUND: Antiadhesive compounds are promising candidates for prevention or treatment of infections. We have investigated the efficacy of such an agent, 3'-sialyllacto-N-neotetraose (NE-1530), given intranasally for prophylaxis of acute otitis media and for effect on nasopharyngeal carriage of bacteria. METHODS: We did a randomised, double-blind placebo-controlled study at one study site. 507 healthy children were randomly assigned either NE-1530 (n=254) or placebo (253) as intranasal sprays twice daily during 3 months. The children were examined by the study physicians once a month and during illness. Treatment efficacy was estimated from Cox proportional hazards model. A sample of nasopharyngeal secretion was taken at every visit for culture of Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. Adverse events were recorded in study diaries. FINDINGS: At least one event of acute otitis media was diagnosed in 108 (43%) of 254 children in the NE-1530 group and in 86 (34%) of 253 children in the placebo group. The efficacy of treatment was negative, -27% (95% CI -68 to 5; p=0.10). The nasopharyngeal carriage of S pneumoniae, H. influenzae, and M. catarrhalis was not affected by treatment, and the adverse event profiles were almost identical for NE-1530 and placebo. INTERPRETATION: NE-1530 did not have a beneficial effect on the occurrence of acute otitis media or on the nasopharyngeal carriage of bacteria in children.