Osteoarthritis (OA) of the knee is a major problem in the elderly. The main outcomes of this disease, pain and disability, relate poorly to its radiographic severity. This study explores reasons why this might be so. The hypotheses were (i) that patellofemoral OA is a significant cause of pain and disability in the community and (ii) that disability in knee OA is primarily due to muscle dysfunction rather than to anatomical changes within the joint. A postal questionnaire asking about knee pain and disability was sent to 2101 men and women aged over 54 registered at a local General Practic. Knee pain positive respondents and age/sex matched controls were invited to attend for quadriceps femoris isometric strength measurements and radiography. An 80.6% response rate to the questionnaire was obtained. Knee pain was reported in 20% of men and 28% of women and was related to disability (relative risk > 2,p< 0.05). 273 symptomatic and 240 controls had x-rays of their knees. Predominant patellofemoral and predominant medial were the commonest compartmental patterns of knee OA (26% males, 49% females; 51% males, 25% females respectively). Both patterns were significantly associated with disability (relative risk > 2.5,p< 0.05). 181 subjects had quadriceps strength assessments. Logistic regression modelling using quadriceps strength, x-ray score, knee pain, age and sex as dependent variables showed that only strength, pain and age were significant predictors of disability (odds ratios 0.88KgF, 2.34, 1.06/year, respectively; p< 0.01). This study confirms that knee pain is common in the community and associated with disability. It shows that patellofemoral compartment involvement is a common manifestation of knee OA and needs to be taken into account in further studies. Finally, quadriceps weakness and knee pain are the most important determinants of disability investigated here. Strategies designed to improve the strength of the population have the potential to reduce a major burden of disability.