Homicide followed by the perpetrator’s suicide is a rare incident in real life. There are many terms that describe this phenomenon – extended suicide, dyadic death, homicide-suicide, post-aggressive suicide etc. These terms are tend to be used as synonyms. However, after their meticulous analysis, they relate to different behaviours. The aim of this study was to investigate, in the aspect of substantive criminal law, the concept of „extended suicide” and to define the perpetrator’s liability for homicide in the aspect of extended suicide. As a result of the analysis of the available sources, not only the features of the phenomenon but also psychological profile of the perpetrator was specified. Moreover, the term extended suicide was defined. Subsequently, based on the information concerning extended suicide, author created qualification of murders by the perpetrator in the case of his failed suicide attempt. The perpetrator of the murder may be liable to criminal prosecution under article 148 § 1, 2 pt. 3 of the Polish Criminal Code. However, due to the fact that the perpetrator most often suffers from mental disorders of varying degrees of severity, the provisions of article 31 § 1 and 2 of the Polish Criminal Code – insanity and diminished capacity – will be applied in this situation.