We report the second case of osteochondritis dissecans (OCD) of the knee in identical twins (bilaterally in one and unilaterally in the other). Fixations with bio-absorbable pins, cylindrical osteochondral graft, and osteochondral mosaicplasty were all successful and bone union was achieved. We considered that genetic factors remain essential even if other factors (particularly repetitive trauma) are mainly responsible for the occurrence of OCD.