Objective. To estimate the direct and indirect arthritis-attributable costs to individuals with disabling hip and/or knee osteoarthritis (OA). Methods. An established population cohort with disabling hip and/or knee OA from two regions of Ontario, Canada was surveyed to determine participant and caregiver costs related to OA, and the predictors of these costs. Results. The response rate was 87.2%. Of 1378 respondents, 1258 had OA (mean age 73.1 yr, range 59-100). Sixty per cent (n^758) reported

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... OA-related costs. Among these individuals, the average annual cost was $12 200 ($CDN in 2002, where $1.00 CDN $0.81 US). Time lost from employment and leisure by participants and their unpaid caregivers accounted for 80% of the total. Men were less likely than women to report costs (adjusted odds ratio 0.54, P_0.0001), but when they did their expenditures were significantly higher (P^0.004). Greater disability was associated with higher costs: compared with individuals with WOMAC total scores _15, those with scores¸55 were 15 times more likely to report costs, and their costs were 3 times greater (both P_0.0001). Both the young (_65 yr) and very old were more likely to incur costs (P_0.0001), and when they did their costs were higher (P_0.001). Conclusion. Costs incurred were mainly for time lost from employment and leisure, and for unpaid informal caregivers. Failure to value such indirect costs significantly underestimates the true burden of OA. Costs increased with worsening health status and greater OA severity. After adjustment, men were less likely to incur costs, possibly due to greater social resources. Patients and methods A population cohort of 2411 individuals aged !55 yr with disabling hip/knee arthritis was established between 1996 and 1998 [3, 16, 17]. In Phase 1, a screening questionnaire was sent to 100% of residents aged !55 yr in two regions (one urban and