Objective. To identify the optimal initial treatment strategy for knee monarthritis in juvenile idiopathic arthritis (JIA) using a decision model and parents' preferences. Methods. We utilized a decision analysis model with Markov states and a 6-month multi-attribute outcome with 7 dimensions pertinent to the treatment decision. The 3 most common treatment strategies for knee monarthritis were compared: nonsteroidal antiinflammatory drugs (NSAIDs) only, NSAID trial followed by intraarticular corticosteroid injection (IACI) if arthritis was not resolved after 2 months, and initial IACI. Probability estimates for the efficacy and adverse effects of NSAIDs and IACIs were derived from a systematic review of the literature. Parents' preferences for the 7 dimensions of the multi-attribute outcome were elicited by a unique hybrid of the time tradeoff and magnitude estimation techniques. These preferences were then combined with the outcomes of the decision analysis to determine an individual's preferred treatment. Results. The NSAID trial strategy may avert IACIs in some patients, but at a cost of continued active arthritis. The number of patients that need to be treated with the NSAID trial strategy to avoid a single IACI compared with the initial IACI strategy is 3.8 with an expected additional cost of 6.7 months of active arthritis. Of the 12 parent subjects, 11 (92%) preferred the initial IACI strategy and 1 preferred the NSAID-only strategy. These preferences were not sensitive to model assumptions or probability estimates. Conclusion. Initial IACI appears to be the optimal treatment strategy for knee monarthritis in JIA. © 2008, American College of Rheumatology.