The vocabulary size of 16- to 30-month-old children (N = 1235) was assessed using the Estonian adaptation of the MacArthur-Bates Communicative Development Inventory: Words and Sentences (ECDI-II). The relationship between children's expressive vocabulary size and different factors of the child and his/her social environment was examined. Results confirm the findings of studies from other languages showing that girls have larger vocabularies than boys, and first-born children are at an advantage compared to later-born children. Children of highly educated parents have bigger vocabularies than children whose parents have a lower educational level; this effect was significant for both maternal and paternal education. It was found that children of less-educated fathers who spend a high amount of time per week (> 40 hours) in daycare have smaller vocabularies than children of fathers with higher educational attainment.