Behavior disorders frequently are associated with mental retardation. The most common interventions involve psychotropics, behavior modification, or both. Etiologically based treatments, derived from an understanding of underlying disease pathogeneses, are infrequent. However, several genetic diseases are associated with elevated rates of destructive responding. The hyperphenylalaninemias provide an excellent model for alternative interventions that have clear biological plausibility. A literature review is undertaken that provides the biochemical rationale for treatment with a low-phenylalanine diet. Several phenylalanine dietary control studies designed to manage aberrant responding among patients with hyperphenylalaninemia are summarized. Together they provide strong evidence that dietary phenylalanine restriction is the treatment of choice among patients ranging from classic phenylketonuria to milder hyperphenylalaninemia. Corroborating evidence derived from phenylalanine loading, magnetic resonance imaging, and dietary amino acid supplementation studies is presented.