School readiness tests are significant predictors of early school achievement. Measuring school readiness on a large scale would be necessary for the implementation of intervention programs at the community level. However, assessment of school readiness is costly and time consuming. This study assesses the predictive value of a school readiness measure, the Early Development Instrument (EDI), which relies on kindergarten teachers' ratings of children's well-being and social, emotional, and cognitive development. We also compared the predictive value of the EDI with that of a direct school readiness test and a battery of cognitive tests. Data were collected when the children were in kindergarten and a year later, as part of Quebec's Longitudinal Study of Child Development. We found that that the EDI alone explained 36% of the variance in school achievement. The complete battery of measures explained 50% of the variance in early school achievement. Two of the EDI domains (Physical Health and Well-Being and Language and Cognitive Development) contributed uniquely to the prediction of school achievement over and above the cognitive assessments and direct school readiness test. The social and emotional domains of the EDI were at best marginal predictors of school achievement. In spite of this limitation, we conclude that the EDI predicts early school achievement as accurately as measures that take more time and resources to administer. (Contains 5 tables and 1 footnote.) [Additional funding for this article was provided by the Ministere de la sante et des services sociaux du Quebec, the Ministere de la famille et des aines du Quebec, and the Ministere de l'education, du loisir et du sport du Quebec; the Quebec Fund for Research Training and Support to Research, the Fonds de la recherche en sante du Quebec (FRSQ), the Fonds quebecois de la recherche sur la societe et la culture (FQRSC), and the Canadian Institutes of Health Research (CIHR); the Molson Foundation; and theFondation Lucie et Andre Chagnon.]