The longitudinal development of head growth was investigated in girls with Rett syndrome. Measurements were taken retrospectively from different kinds of records. Growth retardation was expressed in standard deviation (SD) scores. In classic types, the mean head circumference fell successively to 2 SD scores below the norm at the age of 4 y. After the age of 8 y it stabilized close to -3 SD scores. The degree of deceleration correlated strongly to the age at which a deceleration of 1 SD score had occurred. In forme fruste variants, the mean head circumference was within normal limits; however, it was significantly below the norm (-0.8 SD scores). Body height deviated to -2 SD scores at the age of 6 y and was highly correlated to decline in head growth. When head growth was related to the severity of motor disability, there was a continuum from almost normal head growth with well-preserved gross motor function and some preserved fine motor function to a marked deceleration in head growth with maximum gross and fine motor disability.