Background: In this proof-of-concept study, we investigated the effect of the predominantly sensory adductor-canal-blockade on established pain in the early post-operative period after total knee arthroplasty (TKA). We hypothesised that the adductor-canal-blockade would reduce pain during flexion of the knee (primary end point) and at rest, as well as reducing morphine consumption and morphine-related side effects (secondary outcomes) compared with placebo. Methods: We enrolled patients scheduled for elective TKA into this double-blind, placebo-controlled, randomised study. During general anaesthesia, we placed a catheter in the adductor canal, and after obtaining pre-block pain scores 30min post-operatively, we injected 30ml of ropivacaine 0.75% (n=21) or saline (n=20) according to randomisation. Clinicaltrials.gov Identifier: NCT01261897. Results: Forty-two patients were randomised, and 41 were analysed. Mean (standard deviation) pain scores during flexion of the knee at 1h post-operatively were 58 (22) mm and 67 (29) mm, ropivacaine and placebo group, respectively (P=0.23) but was significantly reduced in the ropivacaine group when calculated as area under the curve for the interval 1-6h (P=0.02). There were no statistically significant differences regarding pain at rest (P=0.08), morphine consumption (P=0.06), nor morphine-related side effects, apart from nausea (P=0.04). Conclusion: This proof-of-concept study shows promising results regarding the analgesic efficacy of adductor-canal-blockade in post-operative pain treatment after TKA, with a significant reduction in pain during flexion of the knee in the early post-operative period compared with placebo. However, the study was not sufficiently powered to permit final conclusions.