It has been reported that the mesenchymal cells in bone marrow contain progenitor cells of some mesenchymal tissues, such as bone, cartilage, fat, and muscle. Bone marrow mesenchymal cells (BMMC) are thought to be useful for reconstructing injured tissues such as bone and cartilage. In the repair process of osteochondral defect, blood from adjacent bone marrow through the subchondral bone defect is thought to play important roles because it contains progenitor cells and growth factors. To promote the repair of osteochondral defect, cell transplantation has been investigated to make up for insufficient cells from the bone marrow. We have reported that autologous BMMC transplantation promoted the repair of full thickness articular cartilage defects in rabbit knee (J Bone Joint Surg Am 1994). This procedure is easy to perform clinically because the autologous BMMC are easy to obtain and can be culture expanded without losing their capacity for differentiation. We transplanted autologous BMMC for the repair of full-thickness articular cartilage defects in the patellae of a 26-year-old female and a 44-year-old male (Cell Transplantation 2004). BMMC were culture expanded, embedded in collagen gel, transplanted into the articular cartilage defect and covered with autologous periosteum. Six months after the transplantation, clinical symptoms had improved dramatically, the improvement has remained in effect (10 years and 5 months in one case, and 9 years and 7 months in the other), and both patients have been satisfied with the outcome. Histology of both patients revealed that the defect had been repaired with the fibro-cartilaginous tissue. We transplanted BMMC to repair large articular cartilage defects in 24 knees of 24 patients with knee osteoarthritis who underwent a high tibial osteotomy (Osteoarthritis Cartilage 2002). Twelve patients received cell transplantation and the other 12 served as cell free controls. Although the clinical improvement was not significantly different 16 months after surgery, the arthroscopic and histological grading score was better in the cell-transplanted group than in the cell-free control group 8 months after the surgery. A few years ago, we investigated the clinical score for middle follow-up period (63 months), but there was no significant difference. We transplanted BMMC into 9 full-thickness articular cartilage defects of the patello-femoral joints (including 2 kissing lesions) in the knees of three patients, a 31-year-old female, a 44-year-old and a 45-year-old male (J Tissue Eng Regen Med 2007). Six months after transplantation, the patients’ clinical symptoms had improved significantly and the improvements have been maintained over the follow-up periods (17 months to 27 months). Histology of the first patient 12 months after the transplantation revealed that the defect had been repaired with the fibro-cartilaginous tissue. We transplanted BMMC into osteochondral defects in 4 patients with osteochondritis dissecans of the elbow. The mean follow up period was 48 months (33-65). Clinical symptoms have improved significantly and they can play recreational sports. X-ray showed bone formation in subchondral area. MRI revealed that repair cartilage showed the same intensity as normal cartilage. Arthroscopy performed in 2 patients showed that articular surface was smooth like normal articular surface. Histology of the third patient 12 months after the transplantation revealed that the defect had been repaired with the fibro-cartilaginous tissue. Autologous BMMC transplantation can be expected to become an effective method for the repair of articular cartilage defects.