Diacetylated spermine, a polyamine, is clinically used as a tumor marker in human medicine. The purpose of this study was to evaluate the usefulness of diacetylated spermine as a nonspecific tumor marker in dogs. One hundred six dogs were randomly selected and divided into 4 groups (cancer-bearing, n=23; control, n=44; young, n=24; inflammatory, n=15). Urine samples were collected to measure diacetylated spermine and creatinine levels. The diacetylated spermine concentration was determined by enzyme-linked immunoassay, standardized by creatinine concentration. The urine diacetylated spermine concentration was markedly significant elevated in cancer-bearing dogs, as compared to that in control dogs (P=0.0081). Whilst the urine diacetylated spermine level in cancer-bearing group was not markedly different from that in the young group, there was significant different between levels in the control and young groups (P=0.019). No significant difference was observed between the inflammatory and cancer-bearing groups. It is suggested that urine diacetylated spermineis a useful tumor marker in dogs over 2 years of age.