The timing of nocturnal cough and its association with change in ambient temperature was documented in 11 asthmatic children, median age 5.1 years, while they were receiving continuous prophylaxis. Studies were performed in their homes on three nights. A voice activated system with electronic time signal recorded coughing. Ambient temperature was recorded every five minutes throughout the night on a Grant Squirrel data logger. Ten children coughed on 27 nights with a median of six bouts of coughing a night (range 0-272). The cough rate in the two hours after going to bed was significantly higher than the cough rate in the middle of the night (2-4 am or 4-6 am). Peak coughing times were 7-9 pm and 6-8 am. The room temperature was lowest between 5 and 7 am. There were no significant differences between cough rates during periods of rapid temperature change (more than 1 degree C an hour). The timing of nocturnal cough observed in this study differed from the known basophase of circadian cycles described for adults and children.