Noting that many American children enter elementary school without a critical foundation of informal mathematical knowledge, this study examined the impact of a prekindergarten mathematics curriculum grounded in research on early mathematical cognition. A project-developed instrument, the Child Math Assessment, was used on a pre-post basis to examine the informal mathematical knowledge of children. Participating in the study were 83 children enrolled in five preschool classrooms serving middle-income families. The intervention group of 41 children received the preschool mathematics curriculum and were assessed at the beginning and end of their prekindergarten year. The comparison group of 42 children did not receive the curriculum and were similarly assessed. The curriculum was delivered by means of teacher-guided small group activities, a mathematics learning center, and computer activities. Curricular units were: (1) enumeration and number sense; (2) arithmetic reasoning; (3) spatial sense; (4) geometric reasoning; (5) pattern sense and unit construction; (6) logical reasoning; and (7) nonstandard measurement. The results revealed that children in the intervention group possessed a variety of informal mathematical skills and abilities at the beginning of the prekindergarten year, but many of these abilities were not highly developed. By the end of the year, the mathematical knowledge of children in the intervention group had developed significantly, and was more extensive than the knowledge of children in the comparison group. (Author/KB)