OBJECTIVE The aim of the present study was to compare the serum level of COMP in both subsets of Systemic sclerosis (SSc) as a marker of arthritis and reveal an associated subclinical RA overlap and a relation to clinical, laboratory and radiological findings in SSc. METHODS Forty adult SSc patients were included in the study and grouped into the two subsets diffuse (dSSc) and limited (lSSc) SSc. Their mean age was 40 ± 9 years. Thorough history taking and clinical examination were performed to all patients. Skin thickness was scored according to the modified Rodnan skin score method (MRSS). The disease activity was assessed by measuring the Medsger severity score. The joints were extensively examined and the tenderness counted according to the Ritchie articular index (RAI). Relevant laboratory and radiological investigations were carried out. The serum COMP level was determined by ELISA. RESULTS The serum COMP was significantly higher in the SSc patients compared to the control and obviously higher in the dSSc compared to the lSSc patients. The level of COMP was higher in the females and significantly higher in the SSc patients with arthritis (56.5 ± 6.8 ug/ml) compared to those without (34 ± 8.3 ug/ml) (P 0.000). CONCLUSIONS The COMP level may become a nonspecific but useful marker for joint involvement in SSc patients to identify patients at risk of joint damage and developing SSc-RA overlap syndrome even with mild arthritis.