Serum-theophylline concentration was measured in 486 ambulatory patients from 407 referring practitioners. They had been on maintenance treatment of 350 mg theophylline (Bronchoretard) twice daily. The concentration was within therapeutic range (8 to 20 mg/l) in 53% of patients, while it was below it in 43% and above in 4%. About half the patients receiving this dose would thus need individual adaptation of dosage, guided by clinical findings and serum concentration, in order to achieve optimal use of the drug's prophylactic and therapeutic potential. It is concluded that (1) theophylline dosage based entirely on standard dose will lead to an unacceptably high percentage of under-dosing; (2) schematic consideration of body-weight, sex, age, living habits, and accompanying diseases proved to be unreliable and insufficient for individual dose determination; (3) dose adaptation to individual clinical situations and serum concentration is necessary in theophylline treatment of ambulatory patients and, in the light of present technical facilities, is cost-effective.