Objective: The extent and factors associated with knee pain fluctuation are not well-known. We evaluated the prevalence, correlates, and association with function of consistency of knee pain. Design: Participants of the Multicenter Osteoarthritis (MOST) Study, a cohort of individuals with or at high risk of knee osteoarthritis (OA) had baseline knee X-rays, questionnaires, and a question about frequent knee pain (FKnP) (pain on most of the past 30 days) at two time points: a telephone screen and a later clinic visit. We computed the prevalence of inconsistent knee pain (positive answer to FKnP question at only one time point) and consistent knee pain (positive answer to FKnP question at both time points). We evaluated the association of consistency of FKnP with a number of sociodemographic factors, pain severity, and function. Results: There were 2940 participants with complete data (5867 knees) [mean age 62, mean body mass index (BMI) 30.7, 60% female]. Of those, 2977 knees had pain, with 43% having inconsistent and 57% having consistent knee pain. Those with radiographic OA [odds ratio (OR) 0.46], depressive symptoms (OR 0.73), and widespread pain (OR 0.68) (all P< 0.05) were less likely to have inconsistent compared with consistent knee pain. Pain, function, and strength were significantly better in persons with two knees that had inconsistent compared with consistent pain. Conclusions: A substantial proportion of persons with knee pain have inconsistent knee pain, associated with better physical function and strength (adjusting for pain severity). Such pain may be suggestive of an earlier stage of disease. © 2010 Osteoarthritis Research Society International.