a variety of studies involving schizophrenic patients have shown the occurrence of severe disabilities related to perception and cognition. most diagnostics rely on alterations that reveal faulty reasoning. our studies evaluate altered visual form and size perception in schizophrenic and major depression patients using 24 paintings by salvador dalí. we conducted two studies comparing experimental and control groups and found in both cases that schizophrenic patients first saw forms 1.5-3.0 times larger than controls. in another study with major depression patients, we show the absence of alterations in size perception during aggravated depressive symptoms. these effects on size perception can contribute as markers in the precocious diagnostic of the onset of positive symptoms in schizophrenia. we suggest that these paintings by dalí can be employed as a tool to assess the state of visual form and size perception in schizophrenic patients, and therefore prevent cognitive deterioration by introducing early treatment.