This cross-sectional study aimed to explore the nature and the patterns of lexical growth in content and language integrated learning (CLIL) and non-CLIL learners across three successive academic years. To pursue the purpose of the study a total of 110 female students of six classes in 3rd, 4th, and 5th grades in both bilingual and monolingual schools in Tehran, Iran, were selected. The experimental group included 50 students, while the control group included 60 students. Receptive and productive vocabulary sizes of three grades from both environments were compared using the Vocabulary Levels Test (VLT) and the Productive Vocabulary Levels Test (PVLT). The results indicated that learners' receptive and productive vocabulary sizes in both CLIL and non-CLIL groups are below 1,000 words, according to the results of the 2k VLT and 2k PVLT tests. In addition, both CLIL and non-CLIL groups showed a gradual increase in their receptive and productive vocabulary growth from third grade to fourth and fifth grades. Finally, across all three grades, the CLIL group showed a higher increase in their receptive and productive vocabulary growth than the non-CLIL group. The results of the study have implications for bilingual schools, language institutes, and language teachers and learners.