A prospective study on acute, uncomplicated measles infection was carried out in 59 patients hospitalized for an ordinary measles infection. A clinical and serological diagnosis of an acute measles infection was made in all cases. Serial serum and cerebrospinal fluid (CSF) specimens were taken two to five times from each patient and tested for pleocytosis, albumin, and measles-specific antibodies. Pleocytosis was found in 18 patients (30%), usually shortly after the onset of rash. Nine patients had antibodies against measles virus in their CSF. Six of them seemed to have damage to the blood-brain barrier, but in two cases there was a very high serum antibody titer with a normal serum/CSF ratio. One patient had a local antibody production against measles virus in the central nervous system. Conventional electroencephalography (EEG) was recorded on 22 children, and a separate, quantitative EEG study with two to six consecutive recordings was also performed on a group of nine patients. Moderate or strong slowing of background EEG activity was found in 50% of the patients. In the consecutive recordings, the changes culminated a few days after the onset of rash. No correlation seemed to exist between the changes in the CSF and the age of the patient, on the one hand, and slowing of the EEG, on the other.