The body electrolyte composition of 34 patients with severe chronic obstructive pulmonary disease and without complications or medications known to influence body sodium or potassium was studied. Exchangeable sodium, exchangeable potassium, extracellular water, and total body water were measured using radioisotope dilution techniques. From these values and serum sodium and potassium levels, the intracellular water, intracellular concentrations of sodium and potassium, and factors to estimate lean body mass were calculated. Normal values were predicted for factors that vary with weight, age, and sex--exchangeable sodium, exchangeable potassium, total body water, extracellular water, intracellular water, and estimates of lean body mass--for each subject using regression equations that took into account the variables. For values that do not vary with weight or age--residual sodium and intracellular concentration of potassium--comparisons were made with values from our control subjects. The patients with chronic obstructive pulmonary disease showed a slower rate of potassium exchange than the control subjects, requiring at least 48 hours. There was also an increase in residual sodium composed of intracellular sodium and exchangeable bone sodium. The possibilities that the abnormalities could be ascribed to congestive heart failure, loss of lean tissue, or to a slowly functioning sodium pump were considered.