Although there have been substantial evidences on the usefulness of electrolytes for the diagnosis of disease, the evidences for a direct link between serum sodium and serum potassium in relation to a specific disease are very limited. This study was performed to investigate an association between diseases and Na:K ratios in dogs. From January 1997 to December 1999, a total of 39 cases with an Na:K ratio less than 27 were retrieved from the medical records of Veterinary Medical Teaching Hospital, Seoul National University. Ten dogs (25.6%) had a renal or urinary disease, and six (15.4%) had a parasitism. Other miscellaneous diseases included deep pyoderma, grade III patellar luxation, bacterial pneumonia, diabetes, pancreatitis, and pyometra. The Na:K ratio was significantly lower in dogs with renal failures than those with parasitic diseases (p=0.0735). With the criterion of the Na:K ratio < 27, twenty seven dogs (69.2%) had hyperkalemia, whereas thirteen dogs (33.3%) had hyponatremia. Of 13 dogs with Na:K ratios between 20 and 24, six were diagnosed as a renal or urinary tract disease, two as diabetes, and two as a parasitism. The Na:K ratios of 9 dogs were < 20, being with the most prevalent with the disease of renal failures (55.6%). The serum Na:K ratios were more closely related to serum potassium concentrations (gamma= -0.8710) than serum sodium concentrations (gamma=0.4703). Two dogs with diabetes had an electrolyte pattern of hyperkalemia with normonatremia. Further studies are needed to determine the usefulness of Na:K ratio for diagnosis of hypoadrenocorticism, and to establish a relationship between patellar luxation and electrolyte unbalance.