Attention deficit hyperactivity disorder (ADHD) is a heterogeneous behavioral syndrome resulting with significant functional impairment and characterized by the core symptoms of hyperactivity, impulsivity and inattention. Conduct disorder (CD) consists of a repetitive and persistent pattern of behaviors in which the basic rights of others and major age-appropriate societal norms or rules are violated. Risperidone, a benzisoxazole derivative, binds with high affinity to 5-HT2, dopamine D2, and ά1-adrenergic receptors. This treatment has been shown to be safe and effective for both conduct disorder and comorbid ADHD in children. In this case report, a 4-year-old male who developed penile erections after using risperidone is discussed. A 4-year-old male patient was diagnosed as CD and ADHD in another clinic and risperidone was started with an initial dose of 0.5 mg/day, with increments of 0.5 mg every seven days until 3 mg/daily (mean maximum dosage of 0.2 mg/kg/day), when a therapeutic response was reached. After five months, the daily dose was lowered to 1.5 mg/daily because of satisfactory clinical response. Two weeks after the dosage change, the patient started to suffer from penile erection episodes. These episodes were 5-6 erection episodes frequently in the daytime, lasting about five minutes and were not painful. The patient was not taking another medical agent other than risperidone. No other related medical condition, which will cause penile erection, was detected. After detailed examination, penile erections were thought to be related with risperidone. With the discontinuation of the drug, symptoms of penile erections disappeared and the child fully recovered from the adverse effect. DSM-V defines ADHD with three subtypes: a combined subtype in which all three-core signs are present in the last six months; a predominantly inattentive subtype in which only inattention is present in the last six months and a predominantly hyperactive-impulsive subtype in which only hyperactivity and impulsiveness are present in the last six months. In the DSM-5, CD is defined based on the presence of three of 15 criteria that should have been present in the last 12 months, and of which one must have been present in the past 6 months. Risperidone has been shown to be safe and effective for both conduct disorder and comorbid ADHD in children. Risperidone is associated with hyperprolactinemia, sexual dysfunction and priapism. The mechanism of the prolonged erection is thought to be with alpha-1 adrenergic blockage. Most of the literature about this adverse effect is from adult population and the data from pediatric population is very limited. The pediatric cases are frequently on poly-pharmacy. In our case, he was on mono-pharmacy and interestingly in the improvement stage of the treatment. To our knowledge, this is the smallest pediatric patient in literature with penile erection due to risperidone treatment, especially while the daily dosage was lowered. Therefore, although risperidone is reported to be safe and effective in the pediatric group, clinicians should be careful about this undesirable adverse effect in every stage of the treatment. Keywords: risperidone, penile erection, attention deficit hyperactivity disorder, conduct disorder