We describe a series of 45 patients with episodic fever of unknown origin (FUO) who meet the classic criteria for FUO. Recurrent or episodic fever is defined as cyclical fever with seeming remission of the disease and fever-free intervals of at least 2 weeks. The traditional mix of infections, malignancies, and inflammatory diseases (multisystem diseases) represent only 20% of the causes of episodic FUO, in contrast to 60 to 70% in cases of continuous fever. The "miscellaneous" group of patients with episodic FUO is the most numerous and about half of the cases remain undiagnosed. This spectrum of causative diseases resembles the spectrum of so-called "prolonged" FUO, i.e., a feverish illness lasting at least half a year. Follow-up of the undiagnosed cases revealed a good prognosis. Only 1 patient died and only 1 more required continuous treatment with corticosteroids. Seven of the 21 undiagnosed cases required intermittent short-term treatment with either nonsteroidal anti-inflammatory drugs or corticosteroids. The remaining 14 seem to be "cured". Diagnosis was frequently based on the clinical course and on exclusion of other diseases. Gastrointestinal investigation was useful, but serologic and immunologic tests yielded only minimal benefit. Ultrasonography, computerized tomographic scans, and gallium scintigraphy were diagnostically useful. Invasive procedures are rarely required, and careful ambulatory follow-up is probably the best approach in cases of unresolved episodic FUO.