Purpose: In this study, the authors aimed to compare vocabulary size, lexical composition, and late talking in large samples of Korean and U.S. children ages 18-35 months. Method: Data for 2,191 Korean children (211 children recruited "offline" through preschools, and 1,980 recruited "online" via the Internet) and 274 U.S. children were obtained using the Language Development Survey (LDS). Results: Mean vocabulary size was slightly larger in the offline than the online group, but the groups were acquiring almost identical words. Mean vocabulary size did not differ by country; girls and older children had larger vocabularies in both countries. The Korean-U.S. Q correlations for percentage use of LDS words (0.53 and 0.56) indicated considerable concordance across countries in lexical composition. Noun dominance was as large in Korean lexicons as in U.S. lexicons. About half of the most commonly reported words for the Korean and U.S. children were identical. Lexicons of late talkers resembled those of typically developing younger children in the same sample. Conclusions: Despite linguistic and discourse differences between Korean and English, LDS findings indicated considerable cross-linguistic similarity with respect to vocabulary size, lexical composition, and late talking. (Contains 1 figure and 2 tables.)