We have assessed the effects of intervention on medication compliance in asthmatic children. The intervention comprised both written information about the medications and behavioural strategies effected by the physician. Children were assigned at random to either control (received no intervention) or test (received the intervention) groups. Compliance was assessed by questionnaire. The mean compliance for the test (78.0%; n = 93) and for the control (54.5%; n = 103) groups differed significantly (P less than 0.001; Mann-Whitney U-test). The test group had a better knowledge of asthma and of the medications, and was more satisfied with the physician and with the regimen than was the control group. These variables were also related to good compliance. This study demonstrates that a programme of intervention can significantly improve medication compliance and can be accompanied by increases in the knowledge of, and satisfaction with, treatment.