CCNU (1-[2-chloroethyl]-3-cyclohexyl-1-nitrosurea), lomustine, is an oral alkylating agent in the nitrosourea subgroup. The dose-limiting toxicity of CCNU is neutropenia most frequently documented 7 days after its administration. Use of prophylactic antimicrobials to prevent chemotherapy-related febrile neutropenia (FN) and its associated morbidity and mortality has been well-documented in human oncology, but this information is limited in the veterinary literature. The purpose of this multi-institutional retrospective study was to assess whether antimicrobial prophylaxis reduced the risk of FN approximately 7 days after CCNU administration in tumour-bearing dogs. A secondary goal was to identify risk factors for fever development in neutropenic dogs. Two hundred dogs were included in the study. One hundred and fifty-three dogs (76.5%) were neutropenic at the first post-CCNU recheck. One hundred and six (69.3%) dogs received prophylactic antimicrobials and 47 (30.7%) did not. Of the 106 dogs on prophylactic antimicrobials, 8 (7.5%) developed FN. Of the 47 dogs in the no-prophylactic antimicrobials group, 4 (8.5%) developed FN. Use of prophylactic antimicrobials did not reduce the risk of development of FN (p = 0.84). Older age (>
9 y), lower weight and body surface area, and pre-treatment with chemotherapy or radiation therapy were significantly associated with development of FN (p = 0.009, p = 0.023, p = 0.015 and p = 0.01). Patients with a lower absolute neutrophil count, and a higher VCOG-CTCAE v2 neutropenia grade were also at an increased risk of developing FN (p = 0.01, p <
0.001). Additional studies may help establish guidelines for antimicrobial prophylaxis in dogs treated with CCNU, especially for those at high-risk for FN.