It is well established that L2 acquisition is faster when the L2 is more closely related to the learner’s L1. In the current study we investigated whether language similarity has a comparable facilitative effect in early simultaneous bilingual children. The similarity between each bilingual child’s two languages was determined using phonological and typological scales. We compared the vocabulary size of bilingual toddlers learning different pairs of languages. Results show that the vocabulary size of bilingual children is indeed influenced by similarity: the more similar the languages, the larger the children’s vocabulary.