Knee arthroscopy is one of the most common surgical procedures in orthopaedics and especially in sports medicine. Portal problems and effusion after knee arthroscopy have been reported. The fluid retention within the joint in knee arthroscopy can affect clinical outcomes, but there is no consensus on portal management. The studies of portal management in knee arthroscopy have mainly addressed wound healing and cosmetic problems. There is insufficient information in the literature about whether the irrigation fluid used in this effusion contributes to the process. This study investigates whether arthroscopic irrigation fluid is retained in the joint and whether portal-closure management has an effect on effusion. In this randomized, prospective study, 91 patients (46 [50.5%] sutured-portal group and 45 [49.5%] open-portal group) were included. Suprapatellar knee-diameter measurement and the number of times the dressing was changed were used to assess intra-articular fluid collection. The visual analog scale (VAS) score, Oxford knee score, and Knee Society score were used to evaluate knee problems. All portal wounds in both groups healed without any problems. Superficial or deep infection was not observed in either group. There was no statistically significant difference in VAS score, Oxford knee scores, and Knee Society scores between groups. Although there was a decrease in the knee diameter of both groups between the early postoperative period and first postoperative day, a statistically significant difference in knee diameter was found, especially in the open-portal group. There was a significant difference between the groups in terms of the number of dressings used in the first 24 hours after surgery. Leaving portals open may be effective in preventing intra-articular fluid retention. We thus advise leaving the arthroscopy portals open with just a simple dressing for selected patients.