To relate functional outcomes to mutation type and age at evaluation in patients with Rett syndrome (RTT).We identified 96 RTT patients with mutations in the MECP2 (methyl-CpG-binding protein 2) gene. Chart analysis, clinical evaluation, and functional measures were completed.Among 11 mutation groups, a statistically significant group effect of mutation type was observed for self-care, upper extremity function, and mobility, on standardized measures administered by occupational and physical therapists. Patients with R133C and uncommon mutations tended to perform best on upper extremity and self-care items, whereas patients with R133C, R306C and R294X had the highest scores on the mobility items. The worst performers on upper extremity and self-care items were patients with large deletions, R255X, R168X, and T158M mutations. The lowest scores for mobility were found in patients with T158M, R255X, R168X, and R270X mutations. On categorical variables as reported by parents at the time of initial evaluation, patients with R133C and R294X were most likely to have hand use, those with R133C, R294X, R306C and small deletions were most likely to be ambulatory, and those with R133C were most likely to be verbal.Functional performance in RTT patients may relate to the type of mutation. Knowledge of these relationships is useful for developing appropriate rehabilitation strategies and prognosis.