Prophylaxis using pegfilgrastim is recommended to prevent cabazitaxel-induced neutropenia. We observed GradeB3 neutropenia in a patient after administration of cabazitaxel, despite prophylaxis using pegfilgrastim. To identify the risk factors associated with GradeB3 neutropenia, we retrospectively investigated the records of 10 patients who received prophylaxis with pegfilgrastim after administration of cabazitaxel. They were divided into GradeB3 neutropenia and non-GradeB3 neutropenia groups, and we compared the background data and laboratory values between the 2 groups. A univariate analysis revealed that hypoalbuminemia was significantly observed in patients with cabazitaxel-induced GradeB3 neutropenia. The incidence of GradeB3 neutropenia was significantly high in patients with serum albumin levels<3.6 g/dL. Cabazitaxel has a high rate of protein binding; moreover, serum albumin levels<3.6 g/dL might be associated with high concentrations of unbound cabazitaxel, and thus an increase in the incidence of GradeB3 neutropenia. Therefore, hypoalbuminemia at the time of administration of cabazitaxel may be a risk factor related to the development of GradeB3 neutropenia.