This was a retrospective clinical study based on the examination of coroners' files from Quebec for January 1991 through May 1998. From these files researchers identified 34 cases of victims who were killed by their mothers. Most victims were less than 6 years of age, and there were several cases of the murders of multiple siblings. Of the 34 victims, 19 (55.9 percent) were male, and 15 (44.1 percent) were female. They ranged in age from approximately 4 weeks of age to 13 years. There were 27 mothers in the sample of perpetrators, and 15 of these women committed suicide after the filicide. The majority of perpetrators were white and of Canadian birth. A psychiatric motive was determined for the actions of 23 of the 27 mothers. Eighteen mothers had a diagnosis of schizophrenia or other psychosis. There were no diagnoses of substance abuse or paraphilia. Almost half of the mothers had previous contact with others regarding their problems, including medical or psychiatric staff. Most offenses occurred in the family home, and the most common method of killing the children was carbon monoxide poisoning, followed by use of a firearm. Based on data from this study, the authors developed a revised classification system that takes into account several characteristics of filicide and associated circumstances. It is flexible and standardized to allow the extraction of subpopulations for research and identification of biological and genetic markers. The authors advise that this attempt to reclassify filicide must be viewed in the context of new research in genetics and identification of genes, as well as the involvement of serotonergic systems in suicide and aggression. This calls for the development of a classification system that would allow for the identification of subgroups with similarities of clinical factors and behavior. The proposed classification instrument must be further standardized to increase its value to researchers and clinicians. Case studies, 3 tables, and 34 references