In a placebo-controlled, double-blind cross-over study of 2 X 3 weeks' duration, twenty-four children with stable asthma who were wheezing during the night, were treated with a single dose of sustained-release theophylline (SRT) taken after supper. The mean serum theophylline levels 4 and 12 hr after dosing were 7.7 and 11.2 mg/l, respectively. Few side-effects were seen. The mean morning peak expiratory flow (PEF) was significantly higher during SRT treatment (244 +/- 11 1/min) than during placebo treatment (207 +/- 121/min) (P less than 0.001). The mean difference between morning and evening PEF was reduced from 20.7 to 8.6% by treatment with SRT (P less than 0.001). Theophylline significantly reduced the severity of attacks of bronchoconstriction during the night as judged by PEF measurement and use of extra bronchodilator treatment per attack. The response to inhaled terbutaline was increased during SRT treatment compared with that in the placebo period, however pre-treatment PEF did differ significantly between the two periods. The number of acute asthma attacks during the night, the number of symptom-free nights and the use of extra bronchodilators during the night were all significantly improved by SRT treatment (P less than 0.001). Seventeen children correctly identified the SRT period whilst six children showed no preference for either period. A single dose of SRT taken after supper is an effective treatment for nocturnal asthma in children.