Simple SummaryCancer is as common amongst pets as it in humans. Chemotherapy can be integrated into treatment regimes for terminally ill pets to attempt to shrink tumours to extend life expectancy, but it does not cure cancer and it can have negative side effects including vomiting, depression and behavioral changes. To date, little research has been undertaken to explore owners’ decisions whether or not to treat their animals with chemotherapy. Seventy-eight dog and cat owners completed an online questionnaire to determine if they would opt for chemotherapy if their pet was diagnosed with cancer, and asked how they thought their pet’s quality of life would be affected. Fifty-eight percent of respondents would not use chemotherapy largely due to their previous experience of it. Seventy-two percent over estimated pet survival time post chemotherapy, with most people believing it would lead to remission or a cure. Owners expected their pets to be less active, sleep more and play less, reducing their quality of life. Common side effects associated with chemotherapy were not rated as acceptable. The results suggest pet owners would benefit from an increased understanding of the positive and negative impacts of chemotherapy when initially discussing treatment options with the veterinary team.AbstractChemotherapy is a commonly integrated treatment option within human and animal oncology regimes. Limited research has investigated pet owners’ treatment decision-making in animals diagnosed with malignant neoplasia. Dog and cat owners were asked to complete an online questionnaire to elucidate factors which are key to the decision making process. Seventy-eight respondents completed the questionnaire in full. Fifty-eight percent of pet owners would not elect to treat pets with chemotherapy due to the negative impact of the associated side effects. Seventy-two percent of respondents over estimated pet survival time post chemotherapy, indicating a general perception that it would lead to remission or a cure. Vomiting was considered an acceptable side effect but inappetence, weight loss and depression were considered unacceptable. Owners did expect animals’ to be less active, sleep more and play less, but common side effects were not rated as acceptable despite the potential benefits of chemotherapy. Based on the results, veterinary teams involved with oncology consultations should establish if clients have prior experience of cancer treatments and their expectations of survival time. Quality of life assessments should also be implemented during initial oncology consultations and conducted regularly during chemotherapy courses to inform client decision making and to safe guard animal welfare.