This case report of a 4-year-old boy has a particularly clear clinical history and pattern of hypomania. Besides hyperactivity and emotional lability, which have previously been described in the offspring of a bipolar parent, the boy displayed pathologically prolonged states of emotional arousal to minimal stimulus, as well as euphoria, overtalkativeness, irritability, episodic states of frenzied activity, and minor depressive feelings. Because the clinical picture was so clear, the identification of possible hypomanic behavior in other young children may be aided.