Post-infectious bronchiolitis obliterans (PBO) is a rare form of chronic obstructive lung disease in children with few data on the pulmonary function outcome and underlying inflammatory process. The aim of this study was to determine the change in lung function over time and to investigate by bronchoalveolar lavage (BAL) the inflammatory characteristics of pulmonary involvement. Eleven Caucasian children with PBO were evaluated to estimate the average rate of change in lung function indices using a mixed model. The differential cytology and lymphocyte subsets of BAL fluid were analyzed. The median follow-up was 10.2 (IQR 3.2-12) years. The estimated forced expiratory volume in 1 sec (FEV1) had a baseline intercept of 57% predicted (62% predicted after bronchodilator) at 10 years of age which fell at a rate of 1.01% per year whereas the estimated forced expiratory flow 25-75 (FEF25-75) had a baseline intercept of 36% predicted (42% predicted after bronchodilator) at 10 years of age which fell at a rate of 1.04% per year. The estimated FEV1/FVC ratio had a baseline intercept of 70% (74% after bronchodilator) at 10 years of age which declined with an average slope of 1.02% per year (-1.10% per year after bronchodilator). Although the baseline and post-bronchodilator level of estimated FVC was abnormal (68% and 69% predicted, respectively) it did not change significantly with time. The median disease duration at BAL evaluation was 3.7 (IQR 0.7-8) years. The percentage differential cell counts were characterized by a significant increase in neutrophils (median 50%, IQR 1-66%), and a slight increase of lymphocytes (median 14%, IQR 7.5-15%). In conclusion, pulmonary function in childhood PBO is characterized by significant airway obstruction which deteriorates over time. The presence of an ongoing inflammatory process could explain the decline in lung function over time.