Objective: Prospective study of well children at risk of bipolarity to identify the frequency and pattern of potentially prodromal symptoms/behaviors for bipolar disorder type I (BPI) disorder. Method: A total of 110 at-risk children with a BPI parent and 112 children with well parents were studied. Ten-year data collection used structured and semistructured annual interviews covering developmental, medical, and behavioral features. Randomized histories for 222 children were submitted blindly for risk rating of bipolarity by a panel of clinicians. Results: Children in the bipolar sample had an overall risk rating of 41% as compared with 16% for control children. Features noted more frequently among the at-risk group were anxious/worried, attention poor/distractible in school, easily excited, hyperalert, mood changes/labile, role impairment in school, somatic complaints, and stubborn/determined. Five additional manic-like behaviors became more evident among at-risk adolescents at the 10-year follow-up: high energy, decreased sleep, problems with thinking/concentration, and excessive and loud talking. Conclusions: The children of a parent with BPI manifested, episodically, miniclusters of potentially prodromal characteristics more frequently than the children of normal controls. None of these children met any of the sets of diagnostic criteria for prepubertal bipolar disorder.