OBJECTIVE To evaluate the ability of the macrophage markers CD163 and CD14 to predict different osteoarthritis (OA) phenotypes defined by severity of joint inflammation, radiographic features and progression, and joint pain. METHODS We evaluated 2 different cohorts totaling 184 patients with radiographic knee OA. These included 25 patients from a cross-sectional imaging study for whom there were data on activated macrophages in the knee joint, and 159 patients (134 with 3-year longitudinal data) from the longitudinal Prediction of Osteoarthritis Progression study. Multivariable linear regression models with generalized estimating equations were used to assess the association of CD163 and CD14 in synovial fluid (SF) and blood with OA phenotypic outcomes. Models were adjusted for age, sex, and body mass index. P values less than or equal to 0.05 were considered significant. RESULTS SF CD14, SF CD163, and serum CD163 were associated with the abundance of activated macrophages in the knee joint capsule and synovium. SF CD14 was positively associated with severity of joint space narrowing and osteophytes in both cohorts. SF and plasma CD14 were positively associated with self-reported knee pain severity in the imaging study. Both SF CD14 and SF CD163 were positively associated with osteophyte progression. CONCLUSIONS Soluble macrophage biomarkers reflected the abundance of activated macrophages and appeared to mediate structural progression (CD163 and CD14) and pain (CD14) in OA knees. These data support the central role of inflammation as a determinant of OA severity, progression risk, and clinical symptoms, and they suggest a means of readily identifying a subset of patients with an active inflammatory state and worse prognosis.