Visual analog scale ratings (0-100 mm) by patients (n=38, 26 men, mean age 28 years) and surgeons (n=2) of expectations, satisfaction, current knee function and activity level were compared one year after reconstruction of the anterior cruciate ligament. On average, the surgeons rated the outcome with regard to knee function and activity level as significantly better than the patients (P<0.008). The mean differences ranged from 10 to 19 mm. The discrepancies increased as the patients' ratings got worse. No such differences were seen for expectations or satisfaction. In order to avoid bias, the patient should assess patient-relevant outcomes, such as function and activity level. When comparing and interpreting the results of clinical studies, it should be taken into account that the use of observer-administered questionnaires might introduce a positive bias yielding a significantly better outcome.