Forty-two females with Rett syndrome, aged 2.5 to 47 years, were assessed with the Teller Acuity Cards and a new version of the Fagan test for age 2 years and above, and their parents were interviewed about the children's communication skills. The visual function of the subjects indicated arrested development, and they scored significantly lower on the Fagan test than a normal comparison group. Their visual processing and memory deteriorated somewhat with age, while those of the comparison group showed a slight increase. Both age at onset of Rett syndrome symptomatology and speech measures were inversely correlated with visual processing and memory, indicating that age at recession may have differential consequences for different functions. Among the subjects, persistent looking was associated with low cognitive function. The results have implications for intervention, and demonstrate that the paradigm of preferential looking may be useful in cognitive assessment of females with Rett syndrome.