We present a patient with a severe chondrolysis after arthroscopic partial lateral meniscectomy in a 17-year-old high school basketball player. This is a rare but severe complication after arthroscopic partial lateral meniscectomy. At 7 months after the first operation, a second-look arthroscopy showed numerous cartilaginous debris floating in the knee and a high-grade cartilage damage on the lateral compartment of the tibia. This unexpected complication and a consideration of its etiology are shown.