Reduced functional residual capacity (FRC) is consistently found in obese subjects. In 10 obese subjects (mean (SEM) age 49.0(6)yr, weight 128.4(8)kg, BMI 44(3)kg.m^-2) without respiratory disease, we have examined 1) supine changes in total lung capacity (TLC) and subdivisions; 2) whether values of total respiratory resistance (Rrs) are appropriate for mid-tidal lung volume (MTLV) and; 3) estimated resistance of the nasopharyngeal airway in both sitting and supine postures. The results were compared with those of 13 control subjects with BMI <27kg.m^-2. Rrs at 6Hz was measured by applying forced oscillation at the mouth (Rrs,mo) or the nose (Rrs,na); resistance of the nasopharyngeal airway (Rnp) was estimated from the difference between sequential measurements of Rrs,mo and Rrs,na. All measurements were made when seated and when supine. Obese subjects when seated had a restrictive defect with low TLC and FRC/TLC ratio; when supine, TLC fell 80ml and FRC only 70ml, compared with a mean supine fall of FRC of 730ml in control subjects. Values of Rrs,mo and Rrs,na at resting MTLV in obese subjects were about twice that in control subjects in both postures. Relating total respiratory conductance (Grs = 1/Rrs) to MTLV, increase in Rrs,mo in obese subjects was only partly explained by their reduced MTLV. Rnp was increased in some obese subjects in both postures. Despite the increased extrapulmonary mass load in obese subjects further falls in TLC and FRC when supine were negligible. Rrs,mo at isovolume was increased. Further studies are needed to examine the causes of reduced TLC and increases in Rrs,mo and sometimes in Rnp in obese subjects.