Lomustine (1‐[2‐chloroethyl]3‐cyclohexyl‐1‐nitrosurea, CCNU) is an oral alkylating agent in the nitrosourea subclass that can cause myelosuppression, with neutropenia being the main dose‐limiting toxicity. The aim of this study was to define the frequency of neutropenic events and to identify predisposing risk factors in tumour‐bearing dogs treated with CCNU. Dogs receiving CCNU for various malignancies were identified following a search of hospital databases. Variables analysed for correlation with neutropenia included signalment, body weight, tumour type, CCNU total dose, steroid use, protocol type, use of L‐asparaginase, previous anthracycline administration and use of the drug as first‐line or in the rescue setting. One‐hundred and fifteen cases were included; median age was 7 years (range 1–14 years) and median body weight 27.6 kg (range 3–74 kg). The median CCNU dose was 63.5 mg/m2 (range 27.7–84.9 mg/m2). Neutropenia occurred in 75 cases (65%) and was comprised of grade 1 (28%), 2 (16%), 3 (29.3%) and 4 (26.7%) events. Tumour type (histiocytic sarcoma [HS]), use of CCNU first line, dose >70 mg/m2, absence of co‐morbidities and previous anthracycline administration, were significantly associated with an increased risk of developing neutropenia, including high‐grade events. There was a 1.7% reported mortality rate. When CCNU is used in dogs with HS, first‐line, at a starting dosage >70 mg/m2, in patients with no co‐morbidities or with a history of previous anthracycline administration, there may be an increased risk of developing neutropenia. These data may help guide treatment decisions and minimize treatment delays or potentially life‐threatening complications.