An adult female with congenital Rubinstein-Taybi syndrome (RTS) and severe mental retardation is described, who presented with symptoms of severe over-activity, short attention span, mood lability, and aggressive outbursts in a cyclical pattern, suggestive of recurrent manic-like episodes. These symptoms improved significantly with divalproex (Depakote) monotherapy. Review of the existing studies showed that 10-76% of persons with RTS may be identified with similar behavioral symptoms.