AIM: The primary objective of this prospective cohort study was to determine the effect of weight loss on pulmonary function values in extremely obese children. METHODS: Obese children participated in a 26-week in-hospital or outpatient multidisciplinary treatment programme. Waist circumference was measured and pulmonary function tests were performed at enrolment and after 6 months. RESULTS: The data of 112 children were analysed. The children had a mean age of 14.4 (range 8.5-18.9) years and 62.5% were girls. The mean SD score-body mass index (SDS-BMI) was +3.38 at baseline and +2.91 after the intervention. Lung function improved significantly: functional vital capacity increased by 3.08% (95% CI 1.16% to 5.00%) of the predicted value, forced expiratory volume in 1 s (FEV(1)) by 2.91% (95% CI 1.11% to 4.71%) of the predicted value, total lung capacity by 2.27% (95% CI 1.16% to 5.00%) of the predicted value, and expiratory reserve volume (ERV) by 14.8% (95% CI 8.66% to 20.88%) of the predicted value. The increase in ERV correlated with the reduction in SDS-BMI and with the reduction in waist circumference. FEV(1) did not correlate with the reduction in either SDS-BMI or waist circumference. CONCLUSIONS: Weight loss in severely obese children correlated with an improvement in lung function, especially ERV. The improvement in ERV correlated with the decrease in SDS-BMI and waist circumference.