Abstract Purpose During knee arthroscopy, irrigation fluid from the surgical site accumulates in the sterile reservoir. Whether these fluid collections and also suture material used during the arthroscopic surgical processes show bacterial contamination over time during surgery remains unclear. The purpose of this study was to determine this contamination rate and to analyze its possible influence on postoperative infection. Materials and methods In this study, 155 patients were included. Fifty-eight underwent reconstruction of the anterior cruciate ligament (ACL), 63 meniscal surgery and 34 patients combined ACL reconstruction and meniscus repair. We collected pooled samples of irrigation fluid from the reservoir on the sterile drape every 15 min during the surgery. In addition, we evaluated suture material of ACL graft and meniscus repair for bacterial contamination. Samples were sent for microbiological analysis, incubation time was 14 days. All patients were seen in the outpatient department 6, 12 weeks and 12 months postoperatively and examined for clinical signs of infection. Results A strong statistical correlation (R² = 0.81, p = 0.015) was found between an advanced duration of surgery and the number of positive microbiological findings in the accumulated fluid. Suture and fixation material showed a contamination rate of 28.4% (29 cases). Despite the high contamination rate, only one infection was found in the follow-up examinations, caused by Staphylococcus lugdunensis. Conclusion Since bacterial contamination of accumulated fluid increases over time the contact with the fluid reservoirs should be avoided. Level of evidence IV.