Subjective feeling of fatigue was assessed before and 10, 20 and 30 days after uncomplicated abdominal surgery in 52 patients and after minor otological surgery in 15 patients. In major surgery fatigue increased from 3.0 +/- 0.2 (mean +/- s.e.m.) arbitrary units pre-operatively to 6.1 +/- 0.3, 5.3 +/- 0.3 and 4.1 +/- 0.3 on day 10, 20 and 30, respectively (P less than 0.001). In contrast, after minor surgery fatigue did not change from before surgery (2.1 +/- 0.3) to day 7 (2.9 +/- 0.5) (P greater than 0.05), day 20 (2.3 +/- 0.4) (P greater than 0.4), or day 30 after surgery (2.1 +/- 0.3) (P greater than 0.9). Differences in fatigue between groups were significant postoperatively (P less than 0.001), but not preoperatively (P greater than 0.05). Mean duration of surgery was similar in the two groups. In the abdominal group an increase in postoperative fatigue did not correlate with age, sex, pre-operative degree of fatigue, actual body weight, triceps skinfold thickness (TSF), arm muscle circumference (AMC), anthropometric index (body weight X TSF X AMC) or serum-transferrin, but did slightly with pre-operative body weight related to ideal body weight (r = -0.3, P less than 0.05). Thus, the magnitude of the trauma and not the duration of anaesthesia may be important for occurrence of postoperative fatigue. Furthermore, fatigue after elective abdominal surgery seems to be unpredictable from the pre-operative status of the patient.