Recently, asthma has been reported to be a potential comorbidity of attention deficit hyperactivity disorder (ADHD). It is, therefore, possible to have combined use of medications for both ADHD and asthma although, potential adverse reactions are unknown. The case presented here is that of a 61/2-year-old boy diagnosed as having both ADHD and asthma. He presented with chin trembling after the first administration of OROS methylphenidate and procaterol hydrochloride. This is the first report of an adverse event in patients using a combination of these two types of drugs. It raises an awareness of chin trembling as an adverse effect in patients using a combination of methylphenidate and procaterol hydrochloride, especially among those who are younger and underweight.