This report concerns the analysis of 100 cases of fever of unknown origin, defined according to the criteria of Petersdorf and Beeson. An etiological diagnosis could be reached in 77 cases, distributed as follows: infections, 32 cases; tumors, 14 cases; collagen diseases, 13 cases; various etiologies, 18 cases; and idiopathic, 23 cases of whom 20 had selflimited fever. There were 51 male and 49 female. The incidence of infectious diseases was significantly higher below age 40 when compared with the group of collagen diseases. Recurrent fever was common (43 cases) while continuous fever was unusual (3 cases). Recurrent fever was most frequent in neoplastic diseases, the difference in relation to the other groups being statistically significant (p less than 0.05). Chills, diffuse perspiration and myalgia were more frequent in infectious diseases (p less than 0.01) than in the other groups. Collagen diseases had significantly higher (p less than 0.05) white blood cell counts than neoplastic diseases, while the latter had significantly higher (p less than 0.01) serum lactic-dehydrogenase and alpha 2 globulin levels than infectious and collagen diseases. Hepatosplenomegaly was present more frequently in neoplastic than in infectious diseases. When evaluating diagnostic procedures it was noted that invasive techniques (arteriography, biopsy, laparoscopy, laparotomy) were essential to arrive to the final diagnosis in 47 cases, while noninvasive procedures (serological and immunological tests, bacteriology, conventional radiology, clinical course, and response to therapy) were sufficient in 22 cases. In eight cases the etiology of the fever could only be determined by post-mortem examination.