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To evaluate the relationship between inflammation and pulmonary function, we quantified changes in inflammatory cellular profile, pro-inflammatory cytokines, and pulmonary function in intubated neonates with meconium aspiration syndrome (MAS).Sixteen term infants were studied. Tracheal aspirate fluids, obtained within the first 6, 24, 48, and 96 hr of life were used for measurements of: (1) cellular profile changes; (2) mRNA and protein levels for pro-inflammatory cytokines, IL-1beta, IL-6, IL-8, and TNF-alpha, using RT-PCR and ELISA. Using the same time points as above, we determined mean airway pressure, oxygenation index (OI), alveolar-arterial oxygen gradient, and arterial/alveolar oxygen ratio. Baseline tidal volume and pulmonary compliance were obtained.Birth weight was 3,820 +/- 656 g, gestational age 39.8 +/- 1.4 weeks. Mean airway pressure and OI significantly decreased from the first 6-96 hr of age (P = 0.01, P = 0.027). Cell counts were elevated in the first 6 hr compared to 96 hr (17.4 x 10(6)/ml vs. 1.5 x 10(6)/ml, P0.05). Pro-inflammatory cytokines decreased from the first 6-96 hr: IL-1beta (187 vs. 37 pg/ml, P0.05); IL-6 (3,469 vs. 150 pg/ml, P0.05); IL-8 (16,230 vs. 6,334 pg/ml, P = 0.01).MAS is associated with an inflammatory response characterized by the presence of elevated cell count and pro-inflammatory cytokines which significantly decreased by 96 hr of life. This decrease in lung inflammation has a positive correlation with corresponding decreases in mean airway pressure and oxygenation index, two parameters associated with improved pulmonary function.