This study examined the composition of vocabulary and preschoolers' vocabulary learning in an early literacy program. Fifty-six children with typical achievement, with special needs, and at risk for disabilities participated. Curriculum-based measures (CBM) were used to track children's receptive and expressive vocabulary growth over three curriculum units. Root and rare word compositional analyses described the vocabulary to be learned. Children's performance across units and by word type were compared. Results showed that the preschoolers gained vocabulary and that CBM was sensitive in measuring progress; the rate varied between groups, however, with typically achieving children making significant gains over time. Composition of vocabulary appeared more influential on children's expressive rather than receptive vocabularies, with less growth by groups to recall rare words than root or basic words. Results suggest greater attention to the composition of vocabulary in early literacy programs and the need for CEM to monitor the vocabulary growth of diverse young learners.