Increased activities of inflammatory mediators unopposed by their inhibitors contribute to chronic lung injury and impaired healing in BPD. The deleterious effects of IL-1β, a cytokine involved in inflammation and host defense, are blocked by IL-1 receptor antagonist (IL-1Ra). We proposed that an imbalance of IL-1β and its inhibitors may contribute to the development of BPD. To determine the relative antigen concentrations of IL-1β and IL-1Ra and functional IL-1 activity in lung lavage of infants at risk for BPD, lung lavage was serially obtained from 1 to 28 days from 17 infants with evolving BPD, 13 infants with self-limited RDS, and 6 controls ventilated for nonpulmonary reasons. Overall, there was a high correlation between IL-1β antigen concentration and IL-1 activity (r = 0.82, p = 0.0001). There were no significant differences among the groups for lung lavage variables on day 1. However, in infants who developed BPD, IL-1β antigen concentration and IL-1 activity increased 16- and 61-fold, respectively, during the first week. IL-1Ra remained relatively unchanged during the first month. IL-lβ/IL-1Ra antigen ratio was significantly higher on days 5 (median 0.024) and 7 (median 0.025) compared with day 1 (median 0.004), p < 0.05. These results suggest that a relative imbalance of IL-1β and IL-1Ra may contribute to prolonged inflammation in BPD.