Clinical substage is frequently reported to be prognostic in dogs with lymphoma, yet formal criteria for defining this parameter are lacking. The World Health Organization TNM Classification of Tumors of Domestic Animals simply defines substage as the absence or presence of systemic signs (substages a and b, respectively). We designed a survey to query veterinary oncologists on the criteria they use to determine clinical substage in practice. Gastrointestinal, constitutional and respiratory signs were the most commonly identified clinical factors, with greater than 90% respondents indicating that inappetence, vomiting, diarrhoea, changes in attitude, weakness and dyspnea were integral in assigning clinical substage. Nevertheless, more than three-quarters of respondents also considered metabolic, neurologic and nutritional parameters when making this determination. For most factors, respondents reported mild-to-moderate severity of clinical signs was sufficient for substage b designation.