The Early Childhood Longitudinal Study, Kindergarten Class of 1998-99, began following a nationally representative sample of approximately 22,000 kindergartners in the fall of 1998. Baseline data about these children, their families, and the kindergarten programs were collected from telephone interviews with the children's parents/guardians and from self-administered questionnaires completed by the kindergarten teachers. Data were also gathered during an individual assessment with each child. The study assessed the children's cognitive skills and knowledge, social skills, physical health and well-being, approaches to learning, and the children's family environment. Among the findings were the following: (1) in reading, mathematics, and general knowledge, older kindergartners outperform younger kindergartners; (2) children's performance increases with the level of their mothers' education; (3) kindergartners from two-parent families are more likely to score in the highest quartile than children from single-mother families, although some with single mothers also score in this quartile, and some from two-parent families score in the lowest quartile; (4) for the most part, children exhibit a high incidence of prosocial behaviors; (6) reports of children's problem behaviors vary by race/ethnicity and by whether the teacher or parent is rating the child; (7) kindergartners are generally healthy, although their general health differs by their family type, level of mothers' education, and whether or not the family uses public assistance; (8) in terms of children's approach to learning, both parents and teachers report that girls persist at tasks more often than boys, older kindergartners persist more often than younger ones, and children not at risk persist more often than children at risk for school difficulty; (9) the majority of parents report having more than 25 children's books in the home; and (10) black children are more likely than white, Asian, or Hispanic children to receive before- and/or after-school care. Findings indicate that while first-time kindergartners are similar in many ways, differences exist in children's skills and knowledge in relation to their characteristics, background, and experiences. The study will continue to follow the cohort through their fifth-grade year. (Contains 56 references.) (HTH)