Drug therapy is necessary to treat metastatic and recurrent breast cancer. In Japan, two types of cyclin-dependent kinases (i.e., CDK4/6 inhibitors) are covered under the national healthinsurance system: palbociclib (since December 2017) and abemaciclib (since November 2018). Although there are many reports on the use and side effects of palbociclib in clinical practice, there are few such reports on abemaciclib. Therefore, we investigated the rate of neutropenia and associated background factors in patients taking abemaciclib. Of the 39 patients taking abemaciclib recruited for the study, 22 satisfied the inclusion criteria. Of these, 7 developed Grade 3 or higher neutropenia and had a significantly lower body weight and body mass index (BMI). Furthermore, the white blood cell and neutrophil counts before administration were significantly lower in the expression group compared with the non-expressing group. To predict the development of Grade 3 or higher neutropenia, the receiver operating characteristic (ROC) curve was used to calculate a BMI cut-off value of 23.9 (specificity 85.7%, sensitivity 73.3%, area under the ROC curve 0.80). Based on this cut-off value, BMI was divided into two groups (<23.9 and ≥23.9) and Fisher's exact test was performed. Patients with a low body mass index were more likely to develop Grade 3 or higher neutropenia as a result of increased dosage per kilogram body weight, while among patients with BMI < 25, those with BMI < 23.9 were at high risk of developing Grade 3 or higher neutropenia. Accordingly, caution is required in the treatment of such patients.