Exanthema subitum (roseola infantum) is one of the mildest exanthematous diseases to which infants under 2 years are subject. It possesses, however, considerable medical interest, because its origin is unknown and because there is a good deal of confusion in differentiating it from other exanthematous diseases. The accumulation of clinical evidence within recent years has shown that exanthema subitum is a clinical entity. The lack of information about this disease on the part of the medical profession is probably due to the fact that it is mild and is rarely encountered in hospital practice. The reports in the literature usually concern isolated cases observed in private practice. Under these circumstances the disease must have originated from manifold sources, and the reports are open to the criticism that, owing to the poorly defined onset of the disease, there is no means of knowing how long the infants were ill. Also, one