The binding of theophylline to serum proteins was studied by 18 hour equilibrium dialysis at 37 degrees C in serum from 10 chronic dialysis patients and 6 healthy subjects. The healthy subjects had a mean (+/- S.D.) unbound fraction of 54 +/- 5% while the dialysis patients had 63 +/- 5% unbound (p less than 0.02). For dialysis patients to achieve the same theophylline concentration in plasma water that metabolically normal asthmatics achieve at the "therapeutic" serum theophylline levels of 10-20 micrograms/ml, their total serum theophylline levels should be 8.6-17.3 micrograms/ml.