Behavioral side effects associated with clonazepam may include agitation, aggression, hyperactivity, irritability, property destruction, and temper tantrums. These side effects may be inadvertently confused with other behavioral or psychiatric conditions, especially if exacerbation of existing challenging behavior occurs. This report describes an individual with mental retardation who experienced behavioral exacerbation associated with clonazepam prescribed at 2 mg/day (0.02 mg/kg/day) to treat aggression, self-injurious behavior, property destruction, and screaming, which was measured with a 15-minute partial interval recording measurement method. When clonazepam was reduced and discontinued, these behaviors significantly decreased from 3.1% of intervals (95% confidence band = 1.6% to 4.6%) to 0.1% of intervals (95% confidence band = 0% to 0.1%). Indicators suggesting review by appropriate medical personnel for possible clonazepam behavioral side effects are provided.