The nocturnal worsening of asthma is a common problem that can be difficult to treat. Two different sustained-release theophylline preparations were used to determine (1) if the serum theophylline concentrations (STC) depend on the type and dosing schedule of the preparation, (2) the relationship between STC and the circadian variations in asthma, and (3) the effect of STC on sleep quality and respiratory patterns during the night. In 16 subjects with nocturnal asthma, the STC were significantly higher during the daytime on twice-daily versus once-daily theophylline preparations given at 7 P.M., but the FEV1 values were similar. During the night, the STC were significantly higher with the once-daily regimen, and the awakening FEV1 value was also improved (p less than 0.05). All polysomnographic variables were similar between the two preparations, except that with the once-daily preparation there was a decreased number of hypopneas (p less than 0.05) and fewer minutes below an oxygen saturation of 90% (p less than 0.05). We conclude that patients with nocturnal asthma need their treatment focused on the nocturnal portion of the circadian cycle and that higher STC during this critical time period are beneficial without interfering with sleep quality.