Mega cistera magna (MCM) is a brain development abnormality that occurs in 1% of radiographic images. It occurs when the transverse dimension of the cerebellospinal reservoir is greater than 10 mm. It is the most important reservoir in the cranial cavity. From the front it is limited by the extended medulla, from the rear by the dura mater, and from the top by the cerebellum. Isolated anomalies in the structure of this brain region often do not cause any clinical symptoms. However, more and more attention is paid in research on the participation of the cerebellum in the regulation of cognitive and affective processes. These processes, in turn, may also be related to the regulation of sexual life. The aim of the study is to present the psychosexual functioning of a man diagnosed with MCM, who is suspected of having committed five sexual offenses. The existence of such a structure of the CNS in conjunction with the analysis of the interview, data from the case files and the results of sexological, psychological and psychiatric examinations, made it possible to evaluate the limitation of the respondent's ability to direct his conduct during the alleged acts, while maintaining the ability to recognize their significance.