The physically abused group consisted of 75 children, aged 0-14 years, who were seen by the medical child protection service for South Glamorgan. A control group of 289 children consisted of those who presented to the Accident and Emergency Department at the Cardiff Royal Infirmary after an accidental injury. Findings show that the bruise distribution in physically abused and control children was well described by a negative binomial distribution. There was a different distribution for the three different regions of the body in physically abused children. The bruising frequency and distribution for the physically abused group was different from that of the controls. The study was able to use parameters derived from these distributions to calculate posterior probabilities of physical abuse given a particular pattern of bruises using Bayes' Theorem. The study thus shows that Bayes' Theorem can be used to combine prior probabilities of physical abuse with observed patterns of bruising in given children to obtain posterior probabilities of abuse. This principle has considerable potential for better diagnosis of the physically abused child. 2 tables and 13 references