The objective of this study was to elucidate how cryotherapy after anterior cruciate ligament reconstruction affects intraarticular temperature and clinical results. A prospective and randomized study was performed on 21 knees of 21 patients. The ligament reconstruction was performed by single-incision arthroscopy using autogenous hamstring tendon. On completion of the surgery, thermosensors were implanted in the suprapatellar pouch and the intracondylar notch, and the intraarticular temperature was monitored while the joint was cooled. Cooling was performed in one group at 5 degrees C (N = 7) and in another at 10 degrees C (N = 7), for 48 hours. A control group (N = 7) did not undergo cryotherapy. The cooled groups showed three temperature phases: a low-temperature phase immediately after the ligament reconstruction, followed by a temperature-rising phase and a thermostatic phase. The control group had no low-temperature phase and immediately entered a thermostatic phase. During the low-temperature phase in the treated groups, the temperature of the suprapatellar pouch and of the intercondylar notch were significantly lower than the body temperature. The pain score and the number of times an analgesic had to be administered were both significantly lower in the 10 degrees C group than in the control group. Blood loss was significantly less in the 5 degrees C group than in the control group.