Objective: To map the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) onto the EQ-5D utility index in patients with knee osteoarthritis (OA). Methods: A consecutive sample of patients (n=258) with diagnosed knee OA completed both the WOMAC and the EQ-5D. Regression models with ordinary least squares (OLS) or the censored least absolute deviations (CLAD) as the estimator were used to establish the mapping function. The WOMAC was represented as explanatory variables in four ways: (a) total score; (b) domain scores (i.e., pain, stiffness, and physical function), (c) domain scores plus pairwise interaction terms to account for possible nonlinearities; and (d) individual item scores. Goodness-of-fit criteria included mean absolute error (MAE, the primary criterion) and root mean squared error (RMSE) obtained using an iterative random sampling procedure. Prediction precision was evaluated at individual patient level and at the group level.  Results: The model using OLS estimator and WOMAC domain scores as explanatory variables had the best fit and was chosen as the preferred mapping model. The prediction error at individual level exceeded the maximal tolerance value (i.e. the minimally important difference of EQ-5D) in about 16% of patients. At group level, the width of 95% CI of prediction errors varied from 0.0176 at a sample size of 400 to 0.0359 at a sample size of 100. Conclusions: EQ-5D scores can be predicted using WOMAC domain scores with an acceptable precision at both the individual and group levels in patients with mild to moderate knee OA.