The 24-hour urine protein/creatinine (U[P/C]) ratio was examined in 19 healthy dogs and in 38 dogs with protein-losing nephropathies. A positive correlation existed between the U(P/C) ratio and the 24-hour urine protein output per kilogram of body weight. The U(P/C) ratio in 18 of 19 healthy dogs was less than 0.2; one dog had a ratio of 0.38. The median U(P/C) ratio in dogs with glomerulonephritis (n = 26) and amyloidosis (n = 6) was 5.73 and 22.50, respectively. The median U(P/C) ratio in dogs with chronic interstitial nephritis (n = 6) was 2.89. In the presence of hypoalbuminemia (albumin less than or equal to 1.5 g/dl), a U/(P/C) ratio greater than 1 indicated a nephrotic syndrome. Severe protein-losing nephropathies (ie, severe glomerulonephritis and amyloidosis) were characterized by U(P/C) ratios greater than 10 and urine protein values exceeding 159 mg/kg/day. The 24-hour U/(P/C) may be more sensitive than the 24-hour urine protein output per kilogram of body weight in the detection of mild glomerular disease. Abnormal U(P/C) ratios were present in 5 dogs, 2 with glomerulonephritis and 3 with chronic interstitial nephritis; that had normal or mildly increased 24-hour urine protein output.