Patients with osteoarthrosis suffer from disability and pain. We measured isokinetic and isometric peak torque in 20 women with gonarthrosis (GA) and in 26 healthy controls. Relationships between muscle strength, walking and stair climbing time, pain level and pain disability scores as assessed by the patients using an extensive questionnaire, radiological changes and subchondral sclerosis expressed as bone mineral density (BMD, g/cm2) of the proximal tibia were studied. Precision errors of the torque measurements in both GA patients and controls were approximately 6%. In the GA patients, isokinetic and isometric quadriceps strength was reduced by 40 and 15% (p < 0.01) respectively, and walking and stair climbing time was increased by 30% (p < 0.005). Isokinetic strength was a better predictor of pain level and pain disability scores than isometric strength and radiological stage. Walking time and stair climbing time were not associated with quadriceps strength, pain level, pain disability scores or radiological changes. Subchondral BMD was not predictive of pain. The study suggests that quadriceps strength assessed by isokinetic dynamometry in GA is a reliable measurement. Isokinetic strength was pronouncedly reduced compared to isometric strength and was a more important predictor of pain and pain disability than isometric strength. These findings should be taken into consideration when planning exercise studies and programmes in GA.