This study focused on the role of bilingual versus monolingual preschool education in the development of lexical knowledge in Russian (L1) and Hebrew (L2) among second-generation Russian-Hebrew speaking immigrants in Israel. The study was designed as a longitudinal and comparative study. The lexical knowledge of children was measured three times during the education year. Thirty-two Russian-Hebrew speaking bilinguals (around age three) were selected from bilingual (Russian/Hebrew) and monolingual (Hebrew) preschools. The research design included observation of the preschools, testing of children's lexical knowledge, and a parents' questionnaire. The children's lexical knowledge was measured in both languages by applying the multidimensional approach of different domains: breadth of the lexical knowledge and depth of the lexical knowledge (paradigmatic semantic relations and syntagmatic semantic relations). The results demonstrated that later immersion in L2 and continuing development of L1 within a bilingual educational context not only do not impede the lexical knowledge development in the L2, but can even accelerate its progress. In addition, the differences between the groups in L1 were particularly striking in the paradigmatic semantic relations. Finally, the paradigmatic semantic relations in L1 were significantly and positively correlated with the lexical knowledge in Hebrew in both groups of comparison. (Contains 3 figures and 3 tables.)