Children with ADHD often show symptoms of oppositional defiant disorders (ODD). We investigated the impact of adjuvant risperidone (RISP) to a standard treatment with methylphenidate (MPH) in children with ADHD and symptoms of ODD. Eighty-four children with ADHD and ODD (age: M=8.55; range: 7.28-9.95 years; 73.8% males) took part in a double-blind, randomized, placebo-controlled, clinical trial lasting eight weeks. Participants were randomly assigned either to the MPH+RISP (1 mg/kg/d+0.5 mg/d) or to the MPH+PLCO (1 mg/kg/d+placebo) condition. Symptoms of ADHD, weight, height, and blood pressure were assessed at baseline, and at weeks 2, 4, 6 and 8. Symptoms of ADHD decreased over time, but more so in the MPH+RISP than in the MPH only condition. In the MPH+RISP condition weight, waist circumference and prolactine levels increased over time. Data suggest that adjuvant RISP improved symptoms in children with ADHD and ODD, but weight gain and higher prolactine levels were also observed, which are two alarming side effects. This may become an issue, once children become adolescents, a period of life in which body shape and body self-image are closely linked to self-confidence and peer acceptance. Health care professionals should carefully balance the short-term and long-term costs and benefits of administration of RISP.