We aimed to assess the clinical significance of Krebs von den Lungen-6 (KL-6) in the diagnosis and severity of interstitial lung disease (ILD) in a French cohort of patients with systemic sclerosis (SSc). Serum KL-6 concentrations were measured with chemiluminescent enzyme immunoassay (CLEIA) in 75 SSc patients. Patients were divided into two groups according to the presence of interstitial lung disease (SSc-ILD versus SSc-without ILD) on chest High-Resolution Computed Tomography. Pulmonary function tests, main manifestations and severity of the lung disease (Medsger's severity scale) were collected. KL-6 serum concentrations were significantly higher in SSc-ILD patients than in those without ILD (p < 10(-4)) and were inversely correlated with forced vital capacity, total lung capacity and diffuse lung capacity of carbon monoxide. Serum KL-6 level superior to 872 U/ml appeared as the optimal cut-off value associated with ILD. Patients with a restrictive pulmonary syndrome and dyspnoea had significant higher KL-6 serum concentrations. SSc patients with anti-topoisomerase 1 antibodies had higher KL-6 serum levels than patients with anti-centromere antibodies (p < 10(- 4)). ILD and anti-topoisomerase 1 antibodies were independent factors associated with KL-6 in multivariate analysis. Interestingly, KL-6 serum concentrations positively increased with the patient lung severity. Our study confirms that KL-6 is an accurate biomarker for the diagnosis of SSc-ILD in a French cohort of patients. High KL-6 levels should prompt physicians to assess ILD with pulmonary imaging and pulmonary functions tests. Prospective clinical studies are still required to determine whether levels of KL-6 might predict progression of ILD as well as its usefulness in the timing of therapeutic intervention.