The study subjects consisted of 54 patients with inoperable or recurrent breast cancer who were administered a combination of palbociclib plus endocrine therapy. We examined the onset of neutropenia during the first course of treatment and evaluated the influence that various risk factors had on treatment continuity. Patients with neutropenia Grade≥3 had significantly lower relative dose intensity(RDI) values during the first course of treatment than did patients with neutropenia Grade ≤2. Patients with neutropenia Grade≥3 showed significantly longer treatment to failure than did patients with neutropenia Grade≤2. These results suggest that the degree of neutropenia during the first course of treatment might contribute to treatment continuity and that it is important to improve the curative effect by maintaining appropriate RDI and by continuously administering palbociclib in patients with neutropenia Grade≥3.