In this work, we tried to correlate the usefulness of the Koup nomogram for dosage prediction of continuous theophylline Dm therapy as compared with the usual method of dosification. To do this, a first group of 20 patients (5 with chronic bronchitis and 15 with bronchial asthma) without clinical or biochemical evidence of hepatic or heart disease (3 with smoking habit), were chosen. They were given a loading dose of theophylline 5 mg/kg over 30 min (as aminophylline). A blood sample was then taken after six hours. The result of this value and in accordance with the nomogram determines the individual oral dose of theophylline administered to attain a serum concentration of 10 mg/ml. The oral dose of theophylline, based according to Hendeles was given to another group of 16 asthmatic patients without smoking habit and clinical or biochemical signs of hepatic or heart disease. Two commercial preparations of theophylline were chosen (Theolair or Theodur). We concluded that: 1) Koup's nomogram is useful in estimating the dose requirement of oral theophylline to reach 10 mcg/ml (Css) at steady state, although it could not be useful in greater Css. 2) Among the patients given the dose according to Hendeles, 37.5% showed toxic serum concentrations. 3) Therapeutic serum concentrations could be obtained in the greater number of patients with twice daily doses, independently of the commercial product. 4) A great number of subjects showed the side effects (discomfort of the stomach, irritability, headache) which appeared to have little direct relationship to serum concentration.