Discriminating between malignant pleural effusion (MPE) and benign pleural effusion (BPE) remains difficult. Thus, novel and efficient biomarkers are required for the diagnosis of pleural effusion (PE). The aim of this study was to validate calprotectin as a diagnostic biomarker of PE in clinical settings. A total of 425 patients were recruited, and the pleural fluid samples collected had BPE in 223 cases (53.7%) or MPE in 137 patients (33%). The samples were all analysed following the same previously validated clinical laboratory protocols and methodology. Calprotectin levels ranged from 772.48 to 3,163.8 ng/mL (median: 1,939 ng/mL) in MPE, and 3,216-24,000 ng/mL in BPE (median: 9,209 ng/mL; p < 0.01), with an area under the curve of 0.848 [95% CI: 0.810-0.886]. For a cutoff value of ≤ 6,233.2 ng/mL, we found 96% sensitivity and 60% specificity, with a negative and positive predictive value, and negative and positive likelihood ratios of 96%, 57%, 0.06, and 2.4, respectively. Multivariate analysis showed that low calprotectin levels was a better discriminator of PE than any other variable [OR 28.76 (p < 0.0001)]. Our results confirm that calprotectin is a new and useful diagnostic biomarker in patients with PE of uncertain aetiology which has potential applications in clinical practice because it may be a good complement to cytological methods. The diagnosis of pleural effusion (PE) is a clinical challenge because it can be produced by over 60 diseases 1,2. Nevertheless, in clinical practice the priority is to establish whether the PE is malignant or not. A diagnosis of malignant PE (MPE) implies the presence of advanced-stage tumours and is therefore associated with a poor prognosis 1,3 which requires urgent diagnosis. Thoracocentesis is the first and most simple procedure for the diagnosis of PE 2-4. Unfortunately, while its specificity for establishing malignancy is 100%, the diagnostic sensitivity of pleural fluid (PF) cytological analysis is low. Although the odds of establishing an MPE diagnosis by immunohistochemistry are improved by applying a panel of different antibodies, its diagnostic sensitivity is still only approximately 60% for metastatic PE and less than 30% for mesothelioma 5,6. When the cytology results are negative, more invasive methods such as a pleural biopsy or thoracoscopy are necessary 2,4,7. In this context, more groups are searching for PF biomarkers for malignancy with the aim of avoiding these invasive procedures 8-10. Recent meta-analyses have evaluated the ability of new biomarkers such