This study compared the long-term clinical and radiological outcomes, according to the extent of arthroscopic meniscectomy, of complete and incomplete types of the discoid lateral meniscus. A total of 125 discoid menisci (74 complete and 51 incomplete types) without significant cartilage erosion at the time of surgery were included. The extent of meniscectomy was decided along with tear patterns and the stability of the discoid meniscus. Both clinical and radiological results were evaluated after total or partial meniscectomy. In the complete type of discoid meniscus with less than 5 years of follow-up, the total meniscectomy group showed better clinical results than the partial meniscectomy group. However, with over 5 years of follow-up, there were no differences between the two groups. In the radiological results, there was no significant difference between the two groups during the first 5 years after operation. However, with more than 5 years of follow-up, the partial meniscectomy group showed better results than the total meniscectomy group. In the incomplete-type discoid meniscus, clinical results were better in the partial meniscectomy group regardless of the follow-up periods. In the radiological results, the partial meniscectomy group showed better results for only more than 5 years of follow-up. The long-term prognosis after arthroscopic meniscectomy for the torn discoid lateral meniscus was related to the volume of the meniscus removed.