Benign joint hypermobility syndrome (BJHS) is a syndrome with musculoskeletal pain originating from the increased laxity of the joints and the ligaments. The study was to compare the isokinetic strength of knee extensor/flexor muscles of BJHS patients with healthy controls. Forty patients diagnosed as having BJHS with Brighton criteria and 45 years of age, height and weight-matched healthy controls were recruited for the study. Isokinetic testing was performed with isokinetic dynamometry of Biodex System 3Pro and measurements were recorded at knee extension/flexion pattern concentrically at 60, 180, and 240/s angular velocities. The study group was also evaluated for functional impairment and pain by HAQ and VAS respectively. Knee extensor muscle strength was significantly lower in the patient group compared with the controls. It was hypothesized that the muscle weakness in the study group was related to lengthening of the quadriceps muscle and pain-related inactivity as well as joint instability and proprioception defect.