Down Syndrome children (N=229), aged 1-83 months, from Australia, Canada, and Germany were tested using the Bayley Scales of Infant Development. Test performances on the Bayley's Mental and Motor scales were not dissimilar, leading to the conclusion that young Down Syndrome children from different countries with relatively comparable standards of health and educational provisions develop at a similar rate. As a group, the Down Syndrome children seemed to take about twice as long as normal children to achieve a particular developmental level. The 707 test protocols of the 229 children did not empirically dictate a specific theory of developmental progression. Linear and logarithmic models could be fitted equally well. The same held true with growth functions fitted to an individual subject's longitudinal data. The most striking result was the large variation of test performances in Down Syndrome children at an early age. Standard deviations were about twice as large as expected from a normal sample at equivalent mean performance levels. This result, along with the early fan-like differentiation of growth curves in children tested repeatedly, was felt to imply that Down Syndrome infants are less protected in their early development by biologically based "self-righting processes" than healthy children. (Author/JDD)