Objective. To investigate the construct validity and reliability of US DAS compared with 28-joint DAS (DAS-28) in assessing joint inflammation and in prediction of structural damage in patients with RA. Methods. Ninety patients with active RA were prospectively recruited and followed up during the 6 months of treatment. The patients underwent clinical, laboratory and X-ray assessment, along with blinded power Doppler US (PDUS) and grey-scale (GS) US (GSUS) examination at baseline and 6 months. A subgroup of 25/90 randomly assigned patients underwent MRI examination of their hands at baseline. A PDUS examination of 22 joints and GSUS examination for effusion/hypertrophy (E/H) of 28 joints were performed by two independent examiners, blinded to clinical findings. E/H was qualitatively assessed as absent or present, and PD signal was semi-quantitatively graded from 0 to 3. PDUS score for synovitis in 22 joints and GS score for E/H in 28 joints were included in US DAS calculation. Clinical scoring, PDUS and GSUS inter-observer reliability were evaluated. Results. Strong correlation was found between US DAS and standard assessment of disease activity such as the DAS-28, ESR and CRP levels. Correlation between US DAS and patients' and physicians' visual analogue scale of activity was moderate, whereas correlations of US DAS with Health Assessment Questionnaire - Disability Index (HAQ-DI) and Short Form 36 Health Survey (SF-36) were weak to moderate. US DAS correlated with X-ray, MRI and US parameters and rates of joint damage. Conclusion. US DAS better anticipated future joint damage than standard DAS-28.