A prospective clinical and stress radiographic study comprising 62 patients was performed to assess the value of acute repair of 35 isolated collateral ligament injuries and 27 major injuries. After a follow-up period of 6 years there was no lateral or medical instability in 42 out of 46 patients. This included 31 out of 33 patients in the group with medial collateral ligament injuries. Among the patients with major injuries 16 (60 per cent) had drawer signs, but in only 6 was there significant instability (greater than 6 mm). In the total material it proved difficult to prevent the development of rotatory instability. This was present in 10 patients preoperatively and in 19 at follow-up. However, this instability was on the whole slight and was experienced as annoying by only 4 patients. Measurement of passive instability showed that 55 per cent (34/62) had become completely stable, but this applied to only 30 per cent (8/27) of patients with major injuries. The functional end result was unsatisfactory in 8 patients (13 per cent). Although several patients could compensate muscularly for a marked passive instability, there was a significant correlation between instability and functional loss.