Fundamental aim of these explorative investigations was to describe and make precise characteristic of the homicides committed by offenders who tempore criminis had completely suppressed ability to recognize significance of their actions as well as control their behaviour ( 31 § 1 of the Polish penal code) and to compare them with homicides committed by sane perpetrators. In this research, 62 homicides were investigated, committed by 50 insane perpetrators. Evidence of each case was particularly analyzed with questionnaire contained 252 descriptive data. They concerned perpetrator’s and victim`s profiles, their mutual relationship and broadly enclosed crime circumstances, also modus operandi features, that covered events before, during and after the homicide. Comparative control group were 72 homicides’ cases carried out by 66 sane offenders. Results of the performed investigations showed, that in the way insane perpetrators act on every stage of the homicide, there are specific and repetitive factors, that can be regarded as distinctive features. In actions of insane ofenders some differences can be seen, depending on motivation (pathological or emotional). Moreover, homicides from the insane group differ significantly from acts committed by completely sane perpetrators. These results confirm strict relation between action mode and mental status of the perpetrator.