The purpose of the article is to indicate selected legal and medical aspects of the presence of a witness suffering from schizophrenia in the Polish criminal trial. The author deals with the clinical picture of schizophrenia, ability of a sick witness to testify, and the related procedural consequences.The legal and medical literature, as well as court judgments and expert comments corresponding to the content, have been used in the arguments.The role of the witness requires that their thought processes run properly. Mental disorders may, depending on their type and severity, interfere with these processes.However, not every disease deviation disqualifies testimonies in terms of their value and not all do so to the same extent. Mental illness is one of the most serious mental disorders.One of the most severe forms of psychosis is schizophrenia.The condition for the correct assessment of the testimonies, which is the exclusive right of the procedural body, is a proper image of the witness’s health condition. One of the mechanisms enabling factual findings to be truthful and reaching the material truth is the regulation of Art. 192 § 2 of the Code of Criminal Procedure. The expert does not assess the credibility of the witness's testimony but confirms or excludes the presence of such features of the mental state that may affect the content of the testimony. The testimony obtained cannot be discredited a priori. The credibility of testimony is assessed by the procedural body according to Art. 7 of the CCP. Each case should be approached without prejudice, in a manner marked by cold analysis. The required behaviour of the responsible jurist in this dimension is the activity consisting of extraordinary caution and criticism in interpreting the disturbed state of the witness.