PURPOSE The purpose of this study is to investigate the serial changes in the SP-D concentrations of serum and bronchoalveolar lavage fluid (BALF) in a bleomycin-induced lung injury rat model and compare them with the levels of conventional biochemical markers. MATERIALS AND METHODS Male Wister rats were anesthetized and intratracheally administered bleomycin (1.0 mg/kg). We evaluated the histological changes and SP-D expression of their lung tissues. We also measured the concentrations of SP-D, albumin, and lactate dehydrogenase (LDH) and the numbers of various types of cells in BALF, and the serum levels of SP-D and conventional markers, including LDH, high mobility group box 1, monocyte chemoattractant protein-1, and C-reactive protein. RESULTS The BALF SP-D level increased and peaked on day 3, and then gradually decreased. These variations were significantly correlated with the changes in the BALF albumin level and granulocyte cell count. The serum SP-D level increased from day 5, peaked on day 10, and then gradually decreased until day 28. The changes in the serum SP-D level accurately reflected the extent of the histological changes caused by the lung injury. On the other hand, the serum levels of conventional biomarkers were only elevated for a few days or did not change during the study period. CONCLUSIONS The SP-D level is the most useful marker of the severity of lung injuries. These results suggest that the measurement of SP-D levels is an additional tool for monitoring acute lung injuries in rats.