This retrospective study in 61 cats with malignant lymphomas examined the efficacy of a well-established chemotherapy protocol (cyclophosphamide, vincristine, and prednisolone [COP]) in the Netherlands, a country with a low prevalence of feline leukemia virus (FeLV). Twenty-two cats (36.1%) had mediastinal lymphoma, 11 (18.0%) had alimentary lymphoma, 7 (11.5%) had peripheral lymphoma, 8 (13.1%) had nasal lymphoma, and 13 (21.3%) had miscellaneous lymphoma (including renal lymphoma in 2 [3.3%]). Of the 54 cats that were tested, only 4 (7.4%) were FeLV positive. Complete remission (CR) was achieved in 46 of the 61 cats (75.4%). The estimated 1- and 2-year disease-free periods (DFPs) in the 46 cats with CR were 51.4 and 37.8%, respectively, whereas the median duration of remission was 251 days. The overall estimated 1-year survival rate in all cats was 48.7%, and the 2-year survival rate was 39.9%, with a median survival of 266 days. The median survival time and the 1-year survival rate for mediastinal lymphoma were 262 days and 49.4%, respectively. Siamese cats had a more favorable prognosis for survival and remission than other breeds. Response to therapy in this study was shown to be a significant prognostic indicator. CR is necessary for long-term survival. Cats that did not achieve CR had little chance of survival for longer than 1 year. Young Siamese cats in this study had a greater tendency to develop mediastinal malignant lymphoma at a young age, and all were FeLV negative. In comparison with results reported in other studies with different combination chemotherapy protocols, these are among the highest percentages of remission and the longest survival rates for cats with malignant lymphoma.