Aims: The technique of microfracture for the arthroscopic treatment of articular cartilage (AC) defects has been shown to result in reparative tissue in the defect, however, retrieved tissues have demonstrated þbrocartilagenous material. The objective of this study was to evaluate the tissue types formed in AC defects in an ovine model treated by microfracture with a collagen- GAG-copolymer trilayer matrix consisting of collagen type I, II and III and autologous cultured cells. Methods: Sixteen adult sheep were used in the study following the protocol accepted by the Animal Care Commitee of the University. Two 4.5-mm diameter defects were produced in the medial condyle of the right knee, all AC was removed without penetrating the subchondral bone. In twelve animals microfracture was performed with a curved pick, in four of them without further treatment, in four the defect was covered by the collagen implant alone, and in four by the cell-seeded implant with cultured autologous chondrocytes from the left knee, 4 defects served as controls. After four months the knees were removed, parafþn sections were stained with H & E, Safranin O/fast green, alcian blue, azan, and antibodies to types I and II collagen. Results: All treatment groups showed better þlling of the defects than untreated knees. Histological analysis revealed the biggest amount of hyaline-like tissue in the cell augmented treatment group. Reparative tissue was predominantly þbrocartilage in the other groups. Conclusions: Collagen implants are able to increase the repair of chondral defects in combination with microfracture.