A profile of respiratory complications has been associated with the onset and development of obesity in humans. Similar phenotypes have been routinely demonstrated in genetic animal models of obesity such as the ob mouse (C57BL/6J-Lep^ob ). The objective of the present study was to test the hypothesis that a constellation of respiratory complications are attenuated with leptin (i.e., protein product of the ob gene) replacement. Daily leptin administration during a 6-wk period was conducted to control body weight of mutantob mice similar to genotypic control groups. During the treatment period, repeated baseline ventilatory measurements were assessed by using whole body plethysmography while quasistatic pressure-volume curves were performed to further explore the role of leptin in improving lung mechanics. Diaphragmatic myosin heavy chain (MHC) isoform phenotype was examined to determine proportional changes in MHC composition. In room air, breathing frequency and minute ventilation were significantly (P < 0.01) different amongob treatment groups, suggesting that leptin opposed the development of a rapid breathing pattern observed in vehicle-treated ob mice. Quasistatic deflation curves indicated that the lung volume of leptin-treatedob mice was significantly (P < 0.05) greater relative to vehicle-treated ob mice at airway pressures between 0 and 30 cmH₂O. Diaphragm MHC composition of leptin-treatedob mice was restored significantly (P < 0.05) to resemble the control phenotype. In this genetic mouse model of obesity, the results suggested that respiratory complications associated with the obese phenotype, including rapid breathing pattern at baseline, diminished lung compliance, and abnormal respiratory muscle adaptations, are attenuated with prolonged leptin treatment.