Purpose: Letter knowledge is a key aspect of children's language development, yet relatively little research has aimed to understand the nature of lowercase letter knowledge. We considered 4 hypotheses about children's lowercase letter knowledge simultaneously--uppercase familiarity, uppercase-lowercase similarity, own-name advantage, and frequency in printed English--as well as 3 interactions. Method: Participants were 461 children ranging in age from 3 to 5 years, all of whom attended public preschool programs serving primarily children from low-income homes, who completed a letter naming task. Results: Uppercase familiarity was the strongest predictor of children's lowercase alphabet knowledge; children were more than 16 times more likely to know a lowercase letter if they knew the corresponding uppercase letter. Uppercase-lowercase similarity and frequency in printed English also predicted children's lowercase letter knowledge, as did the interaction between uppercase familiarity and own-name advantage and the interaction between uppercase familiarity and uppercase-lowercase similarity. Conclusions: Findings suggest that transference from uppercase letter knowledge may be a primary mechanism for lowercase letter knowledge and that young children's knowledge of the lowercase alphabet letters is multiply determined.