Background: Serum lactate dehydrogenase (LDH) concentration is an indicator for tissue injury. It may be secreted locally in many conditions. For the first time, this study was performed to investigate the value of LDH level in bronchoalveolar lavage fluid (BALF) in differentiation of benign from malignant single pulmonary nodules (SPNs) and to assess its relationship with serum LDH levels. Methods: This study was a prospective case-control clinical study. It included 59 patients with a SPN and 21 non-smoker healthy adult volunteers as controls. They underwent bronchoscopy with BAL, Transbronchial needle aspiration (TBNA), and transbronchial biopsy (TBB). Both total serum and BAL LDH levels were measured. Results: The range of the BAL LDH levels in the control group was 4.60 -26 mIU/ml, in patients with benign nodule was 6 – 83 mIU/ml, and in those with malignant nodule was 33 -147 mIU/ml. Overall, the mean BALF LDH level was significantly higher in patients with a malignant pulmonary nodule (85.92 ± 28.31) as compared with that of either patients with a benign nodule (19.08 ± 18.35) (p<0.0001) or control group (12.16 ± 6.18) (p<0.0001). No significant difference between the absolute value of BAL LDH level in patients with benign pulmonary nodule and the control subjects was found (p=0.23). There was no correlation between BALF LDH and serum LDH level in patients with SPNs (p=0.595). Conclusion: BALF LDH levels are increased in patients with malignant SPN, but had no significant rise in benign solitary pulmonary nodules. This factor is useful in differentiating benign from malignant SPNs. A low BAL fluid LDH level in a patient with SPN who does not have a tissue diagnosis may be deemed acceptable for observation and follow up. This may save patients the need for operative procedures.