This study examined the degree to which pain catastrophizing and pain-related fear explain pain, psychological disability, physical disability, and walking speed in patients with osteoarthritis (OA) of the knee. Participants in this study were 106 individuals diagnosed as having OA of at least one knee, who reported knee pain persisting for six months or longer. Results suggest that pain catastrophizing explained a significant proportion (all Ps < or = 0.05) of variance in measures of pain (partial r(2) [pr(2)] = 0.10), psychological disability (pr(2) = 0.20), physical disability (pr(2) = 0.11), and gait velocity at normal (pr(2) = 0.04), fast (pr(2) = 0.04), and intermediate speeds (pr(2) = 0.04). Pain-related fear explained a significant proportion of the variance in measures of psychological disability (pr(2) = 0.07) and walking at a fast speed (pr(2) = 0.05). Pain cognitions, particularly pain catastrophizing, appear to be important variables in understanding pain, disability, and walking at normal, fast, and intermediate speeds in knee OA patients. Clinicians interested in understanding variations in pain and disability in this population may benefit by expanding the focus of their inquiries beyond traditional medical and demographic variables to include an assessment of pain catastrophizing and pain-related fear.