The presence of inflammatory cells in the synovial fluid and the general aspect of the knee joint in patients (n=5) with chronic injury (from 1 to 15 years previously) of the anterior cruciate ligament (ACL) were evaluated. The morphology of the vastus lateralis oblique (VLO) muscle of the same leg was also examined. Synovial fluid and muscle biopsies were obtained during arthroscopy for ACL reconstruction. Synovial fluid was processed and stained with May-Grunwald-Giemsa. Sections of frozen muscles were stained with hematoxylin and eosin and processed to detect mATPase. Two patients showed erosive injury and fibrillation of the cartilaginous surfaces of the knee, while the other three patients had injured menisci. Cytological analysis of the synovial fluid showed only mononuclear cells in three patients and mononuclear and polymorphonuclear cells in another. The fifth patient had neither mono- nor polymorphonuclear cells. Although most of the VLO muscle fibers examined had a normal  morphological aspect, bundles of fibers with centralized nuclei, atrophy and diameter irregularity were seen. These results show that in addition to the well known articular dysfunction associated with ACL injury, the synovial fluid also contains monoand polymorphonuclear cells, and there are alterations in the VLO muscle.