Hyperactivity is the term used to refer to children with the hyperkinetic reaction/minimal brain dysfunction syndrome/attention deficit disorder. There are three general theories about the outcome of childhood hyperactivity: (1) the developmental delay therory, which views hyperactivity as a mere maturational lag, as hyperactive children are expected to become normal around adolescence; (2) the continual display theory, which holds that hyperactive persons will display lifelong attention deficits and impulsivity, with possible secondary psychological symptoms; and (3) the eventual decay theory, which expects hyperactive children to develop severe psychiatric syndromes as adults. To test these theories, this study compared aggressive and antisocial outcomes among 22 previously hyperactive males with their 22 nonhyperactive full brothers. All probands and brothers were tested and interviewed between 21-23 years of age. The study found some subjects who were essentially normal as adults, some with a continuation of their basic hyperactive symptomatology, and some with severe psychiatric deviance, thus lending some support to all three theories. Individual and familial/ecological predictors were found to play a role in outcomes. Chief among the individual predictors were childhood intellectual functioning and, to a lesser extent, childhood aggression. Tabular data and 25 references are provided.