We examined 169 deceased persons and 76 homicide–suicide cases reported in Japan’s Kanagawa Prefecture from 1999 to 2011. The relationships between homicide–suicide perpetrators and homicide victims; the numbers of victims; their age, sex, causes and places of death; motivation; and the presence or absence of a suicide note were extracted and examined. The relationship between homicide–suicide perpetrators and homicide victims was examined based on findings from the following: 24 married couples (31%), 22 parents and children aged ≥18 years (29%), 19 parents and children aged ≤17 years (25%), seven families (9%), two couples (3%) and two miscellaneous relationships (3%). The perpetrators comprised 39 men and 40 women, with a mean age of 51.6 years. The victims comprised 39 men and 51 women, with a mean age of 35.4 years. In our study, approximately half of the perpetrators were female, which differed greatly from the reports from Western countries, where most perpetrators were male. Homicide–suicides among married couples accounted for a higher proportion of overall homicide–suicide deaths in Western countries. In Japan, homicide–suicide occurred more frequently with parents and children. Cases in which a mother committed suicide after having killed her young children accounted for a high proportion of these deaths. Because these events occur so frequently in Japan, we recommend making particular efforts to reduce homicide–suicides among mothers and children.