BACKGROUND Validity of European Scleroderma Study Group (EScSG) activity indexes currently used to assess disease activity in systemic sclerosis (SSc) has been criticised. METHODS Three investigators assigned an activity score on a 0-10 scale for 97 clinical charts. The median score served as gold standard. Two other investigators labelled the disease as inactive/moderately active or active/very active. Univariate-multivariate linear regression analyses were used to define variables predicting the 'gold standard', their weight and derive an activity index. The cut-off point of the index best separating active/very active from inactive/moderately active disease was identified by a receiver-operating curve analysis. The index was validated on a second set of 60 charts assessed by three different investigators on a 0-10 scale and defined as inactive/moderately active or active/very active by other two investigators. One hundred and twenty-three were investigated for changes over time in the index and their relationships with those in the summed Medsger severity score (MSS). RESULTS A weighted 10-point activity index was identified and validated: Δ-skin=1.5 (Δ=patient assessed worsening during the previous month), modified Rodnan skin score (mRss) >18=1.5, digital ulcers=1.5, tendon friction rubs=2.25, C-reactive protein >1 mg/dL=2.25 and diffusing capacity of the lung for CO (DLCO) % predicted <70%=1.0. A cut-off ≥2.5 was found to identify patients with active disease. Changes in the index paralleled those of MSS (p=0.0001). CONCLUSIONS A preliminarily revised SSc activity index has been developed and validated, providing a valuable tool for clinical practice and observational studies.