The results of general chemistry profiles of canine patients from Angell Memorial Animal Hospital, Boston, Massachusetts, during 1993 were reviewed for low (<24) serum sodium:potassium (Na:K) ratios. Thirty-seven dogs had low Na:K ratios. The medical records for 34 these patients were available and sufficiently complete to identify conditions that were associated with low Na:K ratios. Of these 34 dogs, 8 (24%) had hypoadrenocorticism, and 14 had renal disease. Twenty-two of the 34 (65%) had Na:K ratios between 24 and 20. Of these 22 dogs, 9 (41%) had renal or urinary tract disease, and 2 (9%) had hypoadrenocorticism. Other diagnoses in this group included pancreatic disease (3), disseminated neoplasia (3), circulatory disturbance (2), pyometra (1), mushroom poisoning (1), and behavior problem (1). Eight of 34 dogs had Na:K ratios between 19.9 and 15. Of these 8 dogs, 4 (50%) had urinary tract disease, 2 had hypoadrenocorticism, 1 had pancreatic disease, and 1 had severe anemia and hypoproteinemia due to severe parasitism. All of the 4 dogs with Na:K ratios <15 had hypoadrenocorticism, and 1 of these 4 had concurrent renal failure. In all dogs, serum potassium concentration was above the laboratory's reference range, but sodium was below the laboratory's reference range in only 18 dogs (53%). Two of the 8 (25%) dogs with hypoadrenocorticism had serum sodium concentrations within the laboratory's reference range. In this population, low Na:K ratios were invariably associated with hyperkalemia but not always with hyponatremia. Although numerous conditions were associated with a low Na:K ratio, renal disease was the most common. Hypoadrenocorticism was present in only 13% of dogs with Na:K ratios between 24 and 15 but was present in all dogs with Na:K ratios <15.