<h4>Background</h4>Neutrophil gelatinase-associated lipocalin (NGAL) is an accurate marker of septic cavitary effusions in people.<h4>Hypothesis/objectives</h4>To evaluate the utility of serum and effusion NGAL concentrations in differentiating septic effusions from effusions caused by other etiologies in dogs.<h4>Animals</h4>Fifty dogs with pleural or peritoneal effusion.<h4>Methods</h4>Ten dogs were prospectively enrolled into each of 5 groups based on effusion etiology: hypoalbuminemia, increased hydrostatic pressure, neoplastic, inflammatory, and septic. The gene discussed is LCN2; the disease is Ascites.