Purpose of the article: The indication of pleural drainage in parapneumonic pleural effusion (PPE) is still controversial. Pleural fluid's (PF) pH is widely used as an indicator of the need for pleural drainage. We hypothesized that PF's lactate will have a high concordance with pH, and thus, may be a valuable tool to determine the need for pleural drainage in pediatric PPE. METHODS We performed a descriptive, prospective study sequentially enrolling those pediatric patients admitted to a tertiary University Hospital with a PPE between 2008 and 2018. Patients were classified in two groups: drainable PPE (pH < 7) and non-drainable PPE (pH > 7). Correlation with the pH, the area under the curve (AUC), and the sensitivity and specificity values for lactate and other parameters (glucose, and LDH) were analysed too. RESULTS 72 patients with a median age of 4 years (interquartile range 2.25-6) were included. Both groups were homogeneous. Lactate levels were higher in the drainable PPE group (p < 0.001), and a strong inverse correlation between pH and lactate was found (r: -0.7; p < 0.001). A lactate cut-off value of 60.5 mmol/L, exhibit an AUC of 0.86 with a sensitivity of 70% and a high specificity (97.9%) to predict a pH < 7. CONCLUSIONS Our data indicates that lactate in PF presents a strong correlation with pH and could potentially serve as a highly specific biomarker of the need for pleural drainage.