Many patients with asthma experience a worsening of symptoms at night and in the early morning, resulting in sleep disruption and possibly altered daily performance. A bronchodilator agent that exerts its maximal effect overnight to control nocturnal symptoms, without a worsening of the disease during the daytime, should improve the treatment of asthma. This investigation examined the efficacy and kinetics of a new chronotherapeutically optimized, sustained-release theophylline formulation administered once daily (OD) in the evening at 8:00 P.M. in comparison with a conventional sustained-release theophylline administered twice daily (TD) at 8:00 A.M. and at 8:00 P.M. in the same dose. After a theophylline clearance study to substantiate normal or slow metabolism of the drug, a dose-titration period, and a 24-h baseline spirometric study of patients not receiving any medication, participants were randomized to 7-day treatment phases with either OD or TD. Each outpatient segment of 6 days of OD and TD was followed by a 24-h inpatient study on Day 7 when serum drug level and spirometric (PEF, FEV1, and FEF25-75) parameters were obtained every 2 h. The conventional TD treatment was associated with a constant serum theophylline level over the 24 h. In contrast, the OD treatment was associated with larger peak-to-trough drug level fluctuation, with higher levels produced overnight and lower ones in the evening at the end of the dosing interval. Compared with the baseline references, both OD and TD significantly improved airflow over the entire 24 h and to a comparable extent. However, between 2:00 and 6:00 A.M., PEF and FEV1 were significantly greater with OD than with TD. The improvement in PEF and FEV1 at this time, because of OD, was correlated with the serum theophylline level. This was not the case for TD. The improvement in airflow over baseline values between 2:00 and 6:00 P.M. was not correlated with theophylline level with either treatment regime. Overall, the chronotherapeutically conceptualized OD treatment administered in the evening resulted in better airflow levels overnight than did the TD regime without loss of airflow in the afternoon.