Psychiatrists have been alleviating the symptoms of depression, schizophrenia, and other diseases and trying to minimize the risk of complications, such as disability, physical illness, and suicide. However, the question is whether we can prevent mental illnesses from developing in the first place, or at least prevent their clinical manifestations so they remain dormant throughout life. Admittedly, this would be an ambitious undertaking, as most psychiatric disorders are multi-determined. For instance, depression might have causal pathways ranging from genetic predisposition to social factors to the use of certain depressogenic drugs. Some of these causes are currently preventable, others are not. Yet protecting individuals from developing a clinical illness, even in the presence of the underlying risk factor, is not a futuristic fantasy. An excellent example is a debilitating intellectual disability and seizure disorder associated with phenylketonuria (PKU). Patients who are diagnosed early and maintain a strict special diet for PKU can have a normal lifespan with normal mental development.