Background: Some of the poor functional outcomes of knee arthroplasty may be due to pain in the contralateral, unreplaced knee. We investigated the relationship between the preoperative pain status of the contralateral knee and the risk of a poor postoperative functional outcome in patients who underwent knee arthroplasty. Methods: We analyzed data on 271 patients in the Multicenter Osteoarthritis Study who had undergone knee arthroplasty since the time of enrollment. Eighty-six percent of these patients were white, 72% were female, and the mean age was sixty-seven years. The severity of pain in the knee contralateral to the one that was replaced was measured before the knee arthroplasty with use of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain scale, with the scores being grouped into four categories (0, 1 to 4, 5 to 9, and 10 to 20). Poor post-arthroplasty function six months or more after surgery was determined with use of the Patient Acceptable Symptom State (PASS) outcome tool and a clinical performance measure of walking speed. We evaluated the relationship between contralateral pain severity and the functional outcomes with use of Poisson regression. Results: Seventy-two (27%) of 264 patients demonstrated poor post-arthroplasty function by failing to attain the threshold PASS score, and seventy-six (30%) of 250 subjects had a slow walking speed. As the pre-arthroplasty pain in the contralateral knee increased, there was a steady increase in the proportion with poor post-arthroplasty function (p < 0.0001 for PASS and p = 0.04 for slow walking speed). Compared with patients who had no pre-arthroplasty pain in the contralateral knee, those in the highest category of contralateral pain severity had 4.1 times the risk (95% confidence interval, 1.5 to 11.5) of having poor self-reported post-arthroplasty function. Patients in whomboth knees had been replaced at the time of outcome collection were less likely to have poor self-reported function than those in whom only one knee had been replaced. Conclusions: Preoperative pain in the contralateral knee is strongly associated with self-reported post-arthroplasty functional outcome and may therefore be a useful indicator of prognosis or a potential target of perioperative intervention. Level of Evidence: Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence. Copyright © 2013 by The Journal of Bone and Joint Surgery, Incorporated.