All admissions for a ten-year period were reviewed to determine the nature of illnesses which appeared at a referral center in the form of a prolonged fever of unknown origin. Ninety-nine patients presented this history. Of these, 40 were found to have serious or lethal diseases, 48 had essentially benign problems, and 11 had no diagnosis established for symptoms which continue. An arbitrary classification shows 29 children had infectious diseases, 11 had collagen diseases, 8 had neoplasia, 4 had congenital or metabolic problems, and 6 had miscellaneous problems. Nine children were found to be completely normal physically, 21 patients had resolving illnesses which were never defined, and 11 children had no diagnosis established and still have major symptomatic difficulty. The spectrum of disease seen in this series differs significantly from that reported in adults with fevers of unknown origin.