We conducted this study to assess long-term melatonin treatment course, effectiveness and safety in children with attention-deficit/hyperactivity disorder (ADHD) and chronic sleep onset insomnia (CSOI). This was conducted by means of a structured questionnaire for the parents. The subjects of this study consisted of participants who previously participated in a randomised clinical trial on melatonin efficacy. The response rate was 93% (94/101). The mean time to follow up was 3.7 yr. No serious adverse events or treatment related co-morbidities were reported. Sixty-five percent of the children still used melatonin daily and 12% occasionally. Temporal discontinuation of treatment resulted in a delay of sleep onset in 92% of the children. Nine percent of the children could discontinue melatonin completely because of improvement of sleep onset insomnia. Long-term melatonin treatment was judged to be effective against sleep onset problems in 88% of the cases. Improvement of behaviour and mood was reported in 71% and 61% respectively. We conclude that melatonin remains an effective therapy on the long term for the treatment of CSOI in children with ADHD and has no safety concerns regarding serious adverse events or treatment related co-morbidity. Discontinuation of melatonin treatment usually leads to a relapse of sleep onset insomnia and in resuming melatonin treatment, even after several years of treatment.