We studied the efficacy of theophylline chronotherapy for nocturnal asthma using the changes of the circadian rhythms in peak expiratory flow rate (PEF). Eight patients with nocturnal asthma were evaluated for the periods with nocturnal symptoms and with an evening dose of theophylline administered daily. Patients recorded their PEF every 4 hours on one of the days (from 7:00 to 23:00 h) in each period. Circadian rhythms in PEF were examined using the single and group-mean cosinor method. Significant circadian rhythms in PEF were observed in the period with nocturnal symptoms. When nocturnal symptoms were present, the bathyphase of PEF was present between midnight and morning. A significant circadian rhythm disappeared or PEF amplitude decreased during theophylline chronotherapy. The circadian rhythm in PEF was altered according to the severity of the asthma. In patients with symptoms present between midnight and early morning, an evening dose of theophylline chronotherapy can be prophylactically used for nocturnal asthma attacks. Consideration of the circadian rhythm and bathyphase of PEF is useful in selecting appropriate chronotherapy for nocturnal asthma.