The efficacy of second dentition, the number of erupted teeth, as an index in assessing school readiness was examined. A series of cognitive, achievement, and social-emotional instruments were administered to a group of 41 first- and 43 second-grade children. The tests used in this study were: Devereux Elementary School Behavior Rating Scale, Metropolitan Readiness Tests, Concept Assessment Kit-Conservation, and Gates-MacGinitie Reading Tests. Scores on these instruments were correlated with the number of erupted second teeth, as determined by a dental hygienist and with chronological age (in months). Second dentition scores were shown to be a far better correlate of overall school readiness than was chronological age. Specifically, second dentition was a useful supplementary readiness index for first-grade boys. The data suggest the importance of biological indices of development in assessing school readiness. Additional areas for future research relating to second dentition were discussed. (Author/CTM)