External hydrocephalus (EH) is a condition in which infants with rapidly enlarging heads are found to have a CT scan that shows widening of the subarachnoid space with mild or no ventricular dilation. In this study, 74 infants with EH associated with a variety of conditions were examined clinically and with CT scans to identify the radiologic features of the condition as well as to understand its clinical and radiologic evolution. Some of these CT scans were compared with those of patients with cerebral atrophy. The characteristic CT findings in EH were bifrontal widening of the subarachnoid space and widened interhemispheric fissure frontally with only mild enlargement of the rest of the subarachnoid space. Occasional mild to moderate ventriculomegaly was also present. In most cases the radiologic findings could be distinguished from those seen in cerebral atrophy, even without the clinical history of an enlarging head. The cases of cerebral atrophy showed prominent sulci throughout without disproportionate bifrontal widening of the subarachnoid space. Two major groups of infants are identified. The largest group (47) consisted of infants with primary megaloencephaly. Megaloencephaly was idiopathic in 40 infants, many of whom (19) met the criteria for benign familial macrocephaly. Others (7) had a clearly defined genetic syndrome. The radiologic abnormalities in these infants usually disappear by 2 years of age. The other major group (14) consisted of infants who sustained CNS or systemic insults such as subdural hematomas (8) or meningitis (3), which cause impaired CSF absorption. Thirteen premature infants with EH were also identified. It is concluded that EH is an age-related self-limited condition occurring in infants with open cranial sutures and that usually resolves without intervention by 2-3 years of age.