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ABSTRACT. Objective. To assess whether serum concentrations of surfactant protein D (SP-D) and Krebs von den Lungen-6 (KL-6), glycoproteins expressed by type II pneumocytes, correlate with the presence of "alveolitis" and measures of lung function in patients enrolled in the Scleroderma Lung Study (SLS). Methods. Serum obtained at baseline screening of patients with systemic sclerosis (SSc, scleroderma) in the SLS was assayed. "Alveolitis" was defined by either bronchoalveolar lavage or thoracic high-resolution computed tomography (HRCT) by SLS criteria. SP-D and KL-6 levels were measured by ELISA in 66 SSc patients (44 with "alveolitis," 22 without "alveolitis") and in 10 healthy controls. These were compared to clinical measures of lung disease and "alveolitis" in the SLS patients. Results. SP-D levels were 300 ± 214 ng/ml (mean ± SD) in the SSc patients compared to 40 ± 51 ng/ml in controls (p < 0.0001). KL-6 levels were 1225 ± 984 U/ml in the SSc patients and 333 ± 294 U/ml in controls (p < 0.0001). SSc patients with "alveolitis" had higher levels of both SP-D and KL-6 than those without "alveolitis." The level of SP-D was 353 ± 219 ng/ml in patients with "alveolitis" and 161 ± 143 ng/ml without "alveolitis" (p = 0.0002). The level of KL-6 was 1458 ± 1070 U/ml in patients with "alveolitis" and 640 ± 487 U/ml without "alveolitis" (p = 0.0001). Receiver operator characteristic curve analysis demonstrated high sensitivity and specificity of both SP-D and KL-6 for the determination of "alveolitis." KL-6 and SP-D were positively correlated with maximum fibrosis scores, but not with maximum ground-glass opacities, on HRCT. Conclusion. Serum levels of SP-D and KL-6 appear to be indicative of "alveolitis" in SSc patients as defined by the SLS, and are significantly higher than in SSc patients without "alveolitis." Serum SP-D and KL-6 may serve as noninvasive serological means of assessing interstitial lung disease in patients wit