We have studied the ventilatory volumes, flow rates, transfer factor and its components (membrane component and capillary volume) in 19 women and 23 children with moderate obesity. The adults showed restrictive defects, but the pulmonary volumes of children were within normal range. Peak flow, flows at 75 and 50% forced expiratory volume, in two groups, normalized for the forced expiratory volume, did not differ between the two groups. The transfer factor was reduced in adults, because of reduction of the alveolar volume, the membrane component was low in both groups; transfer factor and membrane component normalized for functional residual capacity were not different between the two groups. The capillary volume was greater in children than adults, because the excess body weight was greater for the children. In simple obesity, the main alteration is the decrease of distensibility of the chest wall that becomes worse as time goes on and is the cause for the alterations in ventilatory volume, flow and transfer factor.