Bruising is a frequent and often sentinel injury in children who are victims of physical abuse. Children who are evaluated in an emergency department for bruising, which may be due to abuse, present a challenge to physicians; the injuries themselves are medically minor and their severity can only be described qualitatively with photographs. Nonetheless, bruising in an infant or bruising in unusual locations in young children can indicate violence and risk. These children also present a challenge to the Child Protective Services system because the injuries generally resolve quickly without medical treatment and do not result in long-term sequelae. Creatine phosphokinase (CPK) is released from injured muscle and results in increased serum CPK concentrations. We report on a case of isolated bruising due to child physical abuse in which serum CPK concentrations were markedly increased, demonstrating clinically unsuspected rhabdomyolysis. The increased serum CPK concentrations provided important quantitative information about the seriousness of the bruising. A subsequent chart review of children evaluated by our hospital’s child protection team for isolated bruising during a 6-year period demonstrated that there were other children with bruising due to abuse who also had increased serum CPK concentrations. This information suggests that increased serum CPK in children with bruising due to abuse may be more common than previously thought and that this information may have the potential to be used to provide quantitative, objective information about the seriousness of the bruising. We recommend that physicians consider measuring serum CPK in children with bruising due to physical abuse.