In this study, a purpose-built corpus, containing both child-produced and child-directed speech, is used to conduct a longitudinal examination of antonym use among children from the age of two to five years old. Previous research has tended to approach antonym acquisition using either elicitation techniques or corpora of printed adult language. In contrast, this research focuses on the speech of preschool children in naturally-occurring interactions. The discourse functions of antonymy in child-produced and child-directed language are quantified and compared with those identified in adult, written English (Jones 2002). Despite its complexity, Ancillary Antonymy is found to be most common in child-produced speech, even from the age of two, perhaps because of its particular usefulness in structuring ideas and discourse. This study presents a detailed inter-corpus comparison, assesses the discourse functions of antonymy at different stages of childhood, and discusses the correlation between antonym use in child-directed and child-produced speech.