To assess cognitive outcome in patients with tuberous sclerosis complex (TSC) and identify predictive risk factors, we reviewed records of 107 patients who underwent comprehensive neuropsychiatric evaluation. Fifty-seven percent of patients with TSC had normal-range IQ/developmental quotient (DQ). Cognitive outcome was strongly associated with refractory seizures and TSC2 mutation. In TSC, IQ/DQ is bimodally distributed, and more than half of individuals are in the normal range.