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Introduction: It is known from clinical practice that data concerning plasma sodium concentration and its influence on patient prognosis are underestimated. The aim of this study was the evaluation of the prevalence and influence of hyponatraemia on prognosis in patients with lung diseases, particularly with lung cancer., Material and Methods: Retrospective analysis of data obtained from a single Pulmonary Department was performed. A total of 449 patients divided in two groups, were analysed. The first group consisted of all lung cancer patients (n = 290) hospitalized in the analysed period. The second group included patients with hyponatraemia but without diagnosed lung cancer (n = 159). The prevalence of hyponatraemia, including severity (mild, moderate or severe), was evaluated. Histological types of lung cancer as well as comorbidities were taken into account., Results: Hyponatraemia was found in 46.9% of patients with lung cancer, including mild (serum sodium 135-130 mEq/L), moderate (129-125 mEq/L) and severe hyponatraemia (< 125 mEq/L) in 66.9%, 25% and 8.1, respectively. In patients without lung cancer and with recognized hyponatraemia, mild, moderate and severe hyponatraemia were found in 81.8%, 13.2% and 5%, respectively (mainly in obstructive and interstitial lung diseases). Hyponatraemia was observed in 52.6% of patients with non-small cell lung cancer (NSCLC) and in 45.2% of patients with small cell lung cancer (SCLC). There was no statistical significance in prevalence of hyponatraemia between histological types of lung cancer. In patients with lung cancer and hyponatraemia compared to patients with lung cancer but without hyponatraemia, a significant increase of in-hospital mortality was found (28.7% vs. 7.8%, respectively) p < 0.001., Conclusions: Hyponatraemia was a common abnormality found in approximately 50% of lung cancer patients. Hyponatraemia was a significant prognostic factor associated with poor prognosis.