Osteochondritis dissecans (OCD) is an acquired, potentially reversible idiopathic disease of subchondral bone resulting in delamination and sequestration. Although juvenile-type OCD lesions typically appear stable on superficial examination, conservative treatment results in cure in approximate 50% of patients. We hypothesized that juvenile-type OCD lesions exhibit an underlying instability despite stability at the articular surface and this underlying instability might underlie the lack of effectiveness of conservative treatment. In this study, osteochondral cylindrical tissue samples obtained from stable juvenile OCD lesions located at the medial femoral condyle (classical site) were examined. Eight patients with symptomatic juvenile-type OCD at the classical site underwent arthroscopy. Osteochondral cylindrical tissue samples were obtained from the central portion using a biopsy needle. The samples underwent macroscopic and microscopic examination. All cylindrical samples demonstrated macroscopic separation. On microscopic examination, no degenerative changes in articular cartilage and no bone necrosis were observed. Histological examination revealed two distinct patterns in the samples: (1) thick homogeneous hyaline cartilage alone with little fibrous tissue surrounding areas of separation and (2) nearly normal, thin hyaline cartilage above a mixed layer of hyaline cartilage and subchondral trabeculae and fibrous/fibro-cartilaginous tissue at the areas of separation, indicating delayed or nonunion. Pathological findings in stable juvenile OCD lesions indicate an underlying instability at deeper layers of articular cartilage and poor healing at areas of separation. Improved knowledge of the histology of juvenile-type OCD lesions may support surgical treatment. Early marrow stimulation and/or fixation may be the treatment of choice to promote healing even in macroscopically stable juvenile-type OCD lesions.