The authors compared the achievement of children who were enrolled in full-day kindergarten (FDK) to a matched sample of students who were enrolled in half-day kindergarten (HDK) on mathematics and reading achievement in Grades 2, 3, and 4, several years after they left kindergarten. Results showed that FDK students demonstrated significantly higher achievement at the end of kindergarten than did their HDK counterparts, but that advantage disappeared quickly by the end of first grade. Interpretations and implications are given for that finding. (Contains 6 tables.)