To examine the factors that affect the learning of letter names, an important foundation for literacy, we asked 318 US and 369 Brazilian preschoolers to identify each uppercase letter. Similarity of letter shape was the major determinant of confusion errors in both countries, and children were especially likely to interchange letters that were similar in shape as well as name. Errors were also affected by letter frequency, both general frequency and occurrence of letters in children's own names. Differences in letter names and letter frequencies between English and Portuguese led to certain differences in the patterns of performance for children in the two countries. Other differences appeared to reflect US children's greater familiarity with the conventional order of the alphabet. Boys were overrepresented at the low end of the continuum of letter name knowledge, suggesting that some boys begin formal reading instruction lacking important foundational skills.