We analyzed the time-dependent results after Coventry osteotomy in 118 patients (129 cases) with uni-compartmental osteoarthrosis of the knee. The median follow-up was 11.6 years (range 0.7-17 years). Data were noted according to the time since surgery. Group I (> 2 years) consisted of all 129 cases, group II (> 4 years) of 41 cases and group III (> 8 years) of 15 cases. The HSS knee score (max. 100 points) improved from 33.2 +/- 20.4 (range 17-60) to 68.3 +/- 25.3 (range 30-90) in group I, to 54.7 +/- 18.9 (range 29-90) in group II and to 43.7 +/- 20.9 (range 23-85) in group III. The improvement started 4.6 +/- 7.8 months (range 0-60 months) after the operation and persisted for 4 years +/- 37.4 months (range 0-125 months). The functional knee score (max. 100 points) changed from 61.7 +/- 14.1 (range 41-70) to 71.7 +/- 13.1 (range 53-87) in group I, to 70.0 +/- 11.8 (range 54-88) in group II and to 64.2 +/- 8.0 (range 42-90) in group III. The initial loss in knee flexion was 5.6 degrees (range 0 degree-20 degrees) and for extension 1.0 degree (range -5 degrees-25 degrees). Anteroposterior ligament stability (max. 10 points) decreased from 9.2 +/- 2.1 (range 2-10) to 5.6 +/- 1.7 (range 2-9) in group III. Lateral ligament stability (max. 15 points) was relatively constant, from 12.6 +/- 1.9 (range 4-15) to 9.7 +/- 1.9 (range 2-14). Complications included one tibia fracture, one infection, six peroneus pareses, four haematomas and one pseudarthrosis. The mechanical axis was corrected to an average knee valgu2 of 5.2 degrees +/- 7.4 degrees, which deteriorated over time. Radiographic evidence of arthrosis appeared independent of the operation.