The present study investigated developmental changes and predictors of children's intimate and nonintimate knowledge about a best friend. Forty children from the fourth and eighth grades were questioned about a best friend's personality (e.g., what the friend worries about), the friend's preferences (e.g., favorite sports), and other characteristics that are less personal and more external to the friend (e.g., the friend's birthdate). Accuracy of knowledge was determined by comparing children's responses with their friends' self-reports. In addition, several potential predictors of knowledge about a friend were assessed, including children's frequency of contact with the friend, their attraction to the friend, the duration of their friendship, and their cognitive level as judged by the Raven's Standard Progressive Matrices. The results indicated that eighth graders knew more than fourth graders about a friend's personality characteristics and preferences, suggesting that the intimacy of best friendships increases with age. No age change was found for knowledge of a friend's external characteristics. Cognitive level, frequency of contact, and the duration of a friendship predicted significant portions of the variance in external knowledge. Frequency of contact was the only significant predictor of intimate knowledge; cognitive level and attraction to the friend were marginally significant predictors.