【Abstract】 Objective To study the diagnostic value of plasma LDH: pleural effusion ADA ratio in malignant pleural effusion. Methods The levels of LDH in plasma and pleural effusion, plasma LDH: ADA of pleural effusion ratio were analyzed retrospectively in 357 cases of exudative pleural effusion from October 2011 to May 2017. Results The levels of plasma LDH and plasma LDH: pleural effusion ADA were significantly higher in patients with malignant pleural effusion than those with tuberculous pleural effusion and pulmonary inflammatory effusion. The critical value of Plasma LDH: ADA of pleural effusion ratio was greater than 15, the sensitivity of diagnosis of malignant pleural effusion was 095 (95% CI 091099) and the specificity was 094 (95% CI 085098). The positive likelihood ratio was 361 (95% CI 103897) and the negative likelihood ratio was 003 (95% CI 001017). Conclusion Higher plasma LDH and plasma LDH: pleural fluid ADA ratio in patients presenting with exudative pleural effusion can distinguish between malignant and nonmalignant effusion on the first day of hospitalization. The cutoff level for plasma LDH: pleural fluid ADA ratio of＞20 is highly predictive of malignancy in patients with exudative pleural effusion with high sensitivity and specificity.