In this study patients with high endotoxin levels, TLR4 Asp299Gly mutation was associated with less risk of severe bronchiolitis (OR 0.20 CI 95% 0.06–0.64), while in infants with low endotoxin levels there was a tendency of higher risk of severe bronchiolitis (OR 8.96 CI 95% 0.98–81.6). This evidence concerns the gene TLR4 and bronchiolitis.