During a 4-month period, 693 patients undergoing surgical procedures were prospectively studied to investigate the etiology of postoperative fever (≥38°C during 48 hours or more). The overall rate of fever was similar for the three categories of surgical procedures studied (14%, 13.4% and 13.1% respectively after clean, clean contaminated and contaminated surgical procedures). No cause of fever was found in 5%, 2.7% and 1.7% of patients who underwent clean, clean contaminated and contaminated surgical procedures. Several episodes of fever were observed for 12 patients after clean surgery; for 11 of them this was due to infection. The mean interval between febrile episodes was 4.7 days. After clean wound surgery, fever documented as infectious began significantly later (2.7 vs 1.6 days) and lasted significantly longer (5.4 vs 3.5 days) than fever for which no source was determined. Only half of the infections were associated with fever.