Discoid meniscus is a congenital morphological variable anomaly of meniscus which is often asymptomatic. Arthroscopic surgeries on 91 symptomatic lateral discoid menisci of 84 patients were performed at the Department of Orthopedic Surgery, Seoul National University Hospital during the period of Jan. 1987 to Jan. 1994, which equals 19.2% of arthroscopic meniscectomies performed. Follow up was done from minimum of 1 year to maximum of 8 years, with an average period of 3 years and 10 months. Retrospective study was done with the review of clinical records, roentgenograms, MRI, and arthroscopic findings on recorded videotapes to evaluate the clinical and radiological features, results of arthroscopic treatment and possible prognostic factors. The followings are the results: 1. The lateral discoid meniscus encompassed 19.2% of arthroscopic meniscectomies performed at the same period, which was relatively high incidence. Arthroscopic partial and subtotal menis- cectomy resulted in satisfactory results. 2. Since 8 (11%) knees were not diagnosed preoperatively as discoid meniscus due to type being incomplete or torn meniscus displacement, these points should be considered with clinical findings at diagnosis of discoid meniscus. 3. 34.9% of 83 menisci with tear had previous trauma history which showed high vulnerability to tear. Discoid menisci without tears but with grade II intrasubstance increased signal and symptoms were treated with arthroscopic meniscectomy and showed good results. Therefore discoid menisci without tears should be considered of arthroscopic meniscectomy in the presence of clinical symptoms and MRI findings. 4. Lysholm total and pain scores were significantly improved at postop. 1 year and at the final follow up with p<0.05. Factors such as degenerative changes, sex, age, duration of preoperative symptoms, presence of tears and types of meniscectomy gave no significant influence on the results and the prognosis.