Seventeen male patients with chronic obstructive airways disease in remission were separated into two groups according to arterial carbon dioxide tensions. Hypercapnia was associated significantly with hypoxia and increased red cell volume whereas normocapnia was not. Normocapnic patients were significantly lighter than those with hypercapnia. Total body potassium (TBK) measured by the whole body monitor was significantly low in two of the patients studied (P less than 0.005). The mean value for TBK for the normocapnic group as a whole was significantly low (P less than 0.005), but the mean value for the hypercapnic group was not. Serum potassium and erythrocyte potassium concentrations were normal even when TBK was low, and diuretics had no apparent influence on these potassium values. Of four patients (two in the series and two others) who had TBK measured after a recent episode of cor pulmonale, three had significantly low values. The only previous studies using a whole body monitor to measure TBK in chronic obstructive airways disease found no such low values, though other workers estimating exchangeable potassium by isotope dilution techniques had found evidence of gross potassium depletion. It is concluded that low TBK does indeed occur in patients with chronic obstructive airways disease and that gross depletion is more likely to follow an episode of cor pulmonale.