35 sets of developmental scores were obtained on Down syndrome children by administering the Griffiths Mental Developmental Scale and the Bayley Scales of Infant Development in one testing session. The mean age at testing was 14.3 months, with a range of 3-30 months. The developmental scores of the two tests were similar for the 3-7 month-olds, but became significantly different for the 8-30-month-olds, the Griffiths scores being consistently higher. In addition, the Griffiths scores provided more detailed profiles of the developmental status of children with Down syndrome.