Previous studies in humans with breast, colorectal or liver cancer showed that neoplasia was associated with a modification of the blood ratio between Cu-65 and Cu-63 (partial derivative Cu). The aim of the present study was to compare the blood partial derivative Cu of dogs with cancer to healthy controls or dogs with non-oncologic disease. One hundred and seventeen dogs were included in the study (35 dogs with cancer, 33 dogs with non-neoplastic disease, and 49 healthy controls). The partial derivative Cu of dogs with cancer was significantly lower than the ratio of healthy controls (P \textless 0.0001) but not significantly different from dogs with non-oncologic disease. Six dogs with lymphoma were also evaluated after they achieved clinical remission and five out of six had an increase of partial derivative Cu. Further studies are warranted but these results suggest that partial derivative Cu could help in the diagnosis of cancer in a controlled clinical context, and may be a potential biomarker for the follow-up of cancer.