Abusive head trauma (AHT) is a dangerous form of child abuse that can be difficult to diagnose in young children. To determine how frequently AHT was previously missed by physicians in a group of abused children with head injuries and to determine factors associated with the unrecognized diagnosis. Retrospective chart review of cases of head trauma presenting between January 1, 1990, and December 31, 1995. Academic children's hospital. One hundred seventy-three children younger than 3 years with head injuries caused by abuse. Characteristics of head-injured children in whom diagnosis of AHT was unrecognized and the consequences of the missed diagnoses. Fifty-four (31.2%) of 173 abused children with head injuries had been seen by physicians after AHT and the diagnosis was not recognized. The mean time to correct diagnosis among these children was 7 days (range, 0-189 days). Abusive head trauma was more likely to be unrecognized in very young white children from intact families and in children without respiratory compromise or seizures. In 7 of the children with unrecognized AHT, misinterpretation of radiological studies contributed to the delay in diagnosis. Fifteen children (27.8%) were reinjured after the missed diagnosis. Twenty-two (40.7%) experienced medical complications related to the missed diagnosis. Four of 5 deaths in the group with unrecognized AHT might have been prevented by earlier recognition of abuse. Although diagnosing head trauma can be difficult in the absence of a history, it is important to consider inflicted head trauma in infants and young children presenting with nonspecific clinical signs.