This study examines the relative effects of teacher code-switching and English-only instruction on the vocabulary learning of EFL preschoolers (age 5 and 6), along with their attitudes towards language choices in English learning. Seventy-two young EFL learners were engaged in storytelling activities, either in code-switching or English-only conditions, with the only difference between these conditions being the teacher's brief use of children's L1 for vocabulary teaching purposes. The post-test results show that a brief switch to the children's L1 was more effective than English-only instruction for vocabulary acquisition, and this effect was maintained two weeks after instruction. As for the learners' attitudes towards language choices, the preschoolers were found to overwhelmingly prefer to have some L1 use in learning English, and showed negative reactions towards English-only. These results provide the first piece of empirical evidence regarding the issue of teacher code-switching for very young learners.