Objectives - We investigated the effectiveness of homeopathic Arnica montana on postoperative swelling and pain after arthroscopy (ART), artificial knee joint implantation (AKJ), and cruciate ligament reconstruction (CLR).Design - Three randomised, placebo-controlled, double-blind, sequential clinical trials.Setting - Single primary care unit specialised in arthroscopic knee surgery.Participants - Patients suffering from a knee disease that necessitated arthroscopic surgery.Interventions - Prior to surgery, patients were given 1 × 5 globules of the homeopathic dilution 30× (a homeopathic dilution of 1:1030) of arnica or placebo. Following surgery, 3 × 5 globules were administered daily.Primary outcome measures - The primary outcome parameter was difference in knee circumference, defined as the ratio of circumference on day 1 (ART) or day 2 (CLR and AKJ) after surgery to baseline circumference.Results - A total of 227 patients were enrolled in the ART (33% female, mean age 43.2 years;), 35 in the AKJ (71% female, 67.0 years), and 57 in the CLR trial (26% female; 33.4 years). The percentage of change in knee circumference was similar between the treatment groups for ART (group difference Δ = −0.25%, 95% CI: −0.85 to 0.41, p = 0.204) and AKJ (Δ = −1.68%, −4.24 to 0.77, p = 0.184) and showed homeopathic arnica to have a beneficial effect compared to placebo in CLR (Δ = −1.80%, −3.30 to −0.30, p = 0.019).Conclusions - In all three trials, patients receiving homeopathic arnica showed a trend towards less postoperative swelling compared to patients receiving placebo. However, a significant difference in favour of homeopathic arnica was only found in the CLR trial.