Objective To assess the association of clinical and/or serological parameters with ultrasound-defined disease activity in rheumatoid arthritis (RA). Methods Retrospective analysis of 149 consecutive RA patients routinely assessed by sonography of the wrists, metacarpo-phalangeal, and proximal interphalangeal joints. Semiquantitative scoring of synovial hypertrophy/effusion and power Doppler (PD) signals was performed. Sonographic remission was defined by the absence of PD signals. Number of tender and swollen joints, global assessment of disease activity by the physician (VAS-phys) and patient (VAS-pt), C-reactive protein (CRP), erythrocyte sedimentation rate, duration of morning stiffness (MS), simplified disease activity index, disease activity score for 28 joints, clinical disease activity index, and health assessment questionnaires were recorded. Results PD signals as a sign of active disease were observed in 117 (78.5%) RA patients. CRP, erythrocyte sedimentation rate, and MS were higher in patients with PD signals than in patients in remission. CRP >5.0 mg/L (normal values 0-5.0 mg/L), MS >15 minutes, or the combination of both revealed odds ratios of 5.0, 3.0, or 18.9, respectively, to indicate sonography-defined active disease. The other parameters showed no association with the presence or absence of PD-signals. Conclusions Sonography-defined disease activity is associated with CRP and MS, whereas current composite scores and its clinical components did not match this definition.