Injury to the infrapatellar branch of the saphenous nerve has been reported as a complication of arthroscopic examination and surgery of the knee. The authors studied the anatomic distribution of this branch in cadavers, and investigated the incidence of this complication in 68 patients. The results of anatomic study showed that blind puncture is safe within an approximate 30-mm area from the medial margin of the patella at the level of midpatella, and within an approximate 10-mm area from the medial margin of the patellar ligament at the level of the distal pole of the patella. In 30% of examined cadavers, the infrapatellar branch of the saphenous nerve transverses and runs laterally before it crosses the proximal edge of the tibia. Anatomic findings indicated that blind puncture to the knee in a 90 degrees flexion position should be done horizontally and parallel to the articular surface to reduce the incidence of nerve injury. The results of this study of patients who had arthroscopy from 1990 to 1991 revealed a 22.2% incidence rate of sensory disturbances in the area where the infrapatellar branch is distributed. The incidence can be minimized by clarifying the distribution of the infrapatellar nerve branch in relation to palpable landmarks.