Physical aggression directed toward parents by their adolescents is a serious issue both practically and scientifically. In contrast to the extensive literature on other forms of aggression within families (e.g., marital violence, child physical abuse) as well as youth aggression construed broadly, a major gap exists in our knowledge of youth-to-parent physical aggression (YPA). In this study, we analyzed data on three forms of physical aggression (YPA, interparental, and parent-to-youth) from 232 mother-adolescent dyads drawn from a database of families referred for the clinical treatment of emotional and behavioral problems in their adolescent children. Analyses indicated that YPA is prevalent (57% by sons and 49% by daughters in 1 year) and significantly likely to co-occur with interparental and parent-to-youth aggression in the family. Follow-up analyses suggested important sex differences in these relations. (Contains 1 footnote and 5 tables.)