Objective. Although knee pain severity is thought to greatly impact function, the additional contribution of pain in 1 versus 2 knees is not known. We examined the relationship between unilateral versus bilateral pain with low physical functioning at baseline and 30 months while accounting for knee pain severity. Methods. The Multicenter Osteoarthritis Study is a cohort study of people who have or are at high risk for knee osteoarthritis. We defined low physical function as Western Ontario and McMaster Universities Osteoarthritis Index physical functioning scores ≥28 of 68, consistent with poor functional outcome. Incidence and improvement from low physical function were defined as scores that declined below and improved above this threshold at 30 months. We examined the association between pain in 1 or 2 knees with low physical function with risk ratios (RRs) adjusting for known confounders. Results. Of the 2,069 subjects (mean ± SD age 63 ± 8 years, mean ± SD body mass index 31 ± 6 kg/m 2, 63% women), the prevalence of low physical functioning was 50% lower among persons with unilateral pain compared with those with bilateral pain (adjusted prevalence ratio 0.5 [95% confidence interval (95% CI) 0.3- 0.7]). Of those without low physical functioning at baseline, the risk of incidence at 30 months was 30% less for unilateral compared with bilateral pain (adjusted RR 0.7 [95% CI 0.5-1.0]). Of those with low physical functioning at baseline, improvement was 1.7 times more likely for those with unilateral compared with bilateral pain (adjusted RR 1.7 [95% CI 1.3-2.2]). Conclusion. The presence of pain in 1 versus 2 knees provides important information regarding present and future physical functioning. © 2010, American College of Rheumatology.