Meningitis is inflammation of the pia mater, arachnoid and subarachnoid space. Meningitis is caused by infection, by bacteria or viruses. The most common cause of meningitis is viral infection that usually resolves without treatment. Bacterial meningitis is an extremely serious illness which may affect children of any age, but the most susceptible are infants under one month of age and immunosuppressed children. Children who have had bacterial meningitis have a one in four chance of being left with a permanent disability such as deafness, blindness, cerebral palsy, seizures, hydrocephalus or cognitive impairment. The signs and symptoms of meningitis in babies and young children include fever, vomiting, irritability, drowsiness, skin rash and headache. Meningitis is sometimes difficult to diagnose and requires laboratory analysis of cerebrospinal fluid. The role of ultrasound in infants with meningitis is to evaluate for complications. In the early stages of bacterial meningitis the ultrasound findings are usually normal. However as the disease progresses there may be sonographic findings such as echogenic sulci or parenchyma, ventriculitis, ventriculomegaly, venous thrombosis, infarction and abscess formation. Subdural effusions may also be present over the frontal and parietal convexities. Late sequelae include hydrocephalus, brain atrophy and cystic encephalomalacia.