After one year of sustained-release theophylline (SRT) treatment, 19 children with severe perennial asthma participated in a 6-week, placebo-controlled, randomized, double-blind cross-over study. The severity of their asthma was evaluated before SRT-treatment (pre-period) and one year later, both during the theophylline period and the placebo period of the double-blind study. Compared with the pre-period, all observed parameters, except the number of extra bronchodilators required, were statistically significantly improved in the placebo period of the double-blind trial performed after one year. In addition, except for peak expiratory flow measured at home and the number of acute attacks, all parameters were significantly improved during the theophylline period when compared with placebo treatment. During this study, mean serum theophylline level was 13.3 mg/l (range 7.8-19.4 mg/l). The results show that theophylline remains efficient after one year's administration. The much better control of the children's asthma following one year of treatment emphasizes the need for regular re-evaluation of children receiving continuous prophylactic treatment.