Weight gain associated with neuroleptics or antipsychotic treatment is well known by psychiatrists, but is too rarely considered as justifying a specific treatment program. Overweight is a risk factor for somatic disorders and can have a negative influence on self-esteem and self-confidence. This can lead to poor observance, and relapse of psychotic symptoms. Some studies try to describe the weight fluctuations according to the different neuroleptics and taking into account other variables like treatment duration, age or sex. Mechanisms of weight gain are less studied, in spite of evidence that neuroleptics interact with receptors of dopamine, norepinephrine, serotonin, histamine and acetylcholin, all implicated in a way or another, in weight regulation. Antipsychotics, like clozapine and olanzapine, are more concerned with neuroendocrine and neurovegetative interactions, and are responsible for the most severe weight increases. Loxapine and molindone induce weight decreases, and these exceptions are difficult to explain. The paper discusses the clinical and the epidemiological data, and indicates the methodological problems for such studies. Some hypotheses about the pathophysiological aspects of this side effect are made, in regard to growing knowledge about the biological mechanisms of weight regulation. Some solutions for a better consideration and caretaking of patients with such problems or "at risk" treatment are proposed.