PurposeThe combination of cyclin-dependent kinase 4/6 inhibitors and endocrine therapy is a standard treatment for hormone receptor (HR)-positive/human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC); however, their toxicities and financial burden are major issues, especially for prolonged treatment. We investigated fulvestrant plus palbociclib in patients with HR-positive MBC resistant to fulvestrant monotherapy.MethodsPatients who initially received fulvestrant as their first- or second-line endocrine therapy were assigned to group A. Patients with disease progression during fulvestrant monotherapy who subsequently received fulvestrant plus palbociclib were assigned to group B. The primary endpoint was progression-free survival (PFS1) in group B. We set the threshold median PFS of 5 months (null hypothesis).ResultsBetween January 2018 and February 2020 we enrolled 167 patients in group A (January 2018–February 2020) from 55 institutions, of whom 72 subsequently received fulvestrant plus palbociclib and were enrolled in group B. The median follow-up was 23.8 and 8.9 months in groups A and B, respectively. The median PFS in group B (combination therapy) was 9.4 (90% confidence interval [CI]: 6.9–11.2) months (p< 0.001). This was 25.7 (90% CI: 21.2–30.3) months in group A (fulvestrant monotherapy). The TTF in group B was 7.2 (90% CI: 5.5–10.4) months. In thepost-hocanalysis, the median PFS1 in group B among patients with longer-duration fulvestrant monotherapy (> 1 year) was longer than that of patients with shorter-duration monotherapy (≤ 1 year) (11.3 vs. 7.6 months). No new toxicities were observed.ConclusionOur findings suggest that palbociclib plus fulvestrant after disease progression despite fulvestrant monotherapy is potentially safe and effective in patients with HR-positive/HER2-negative advanced MBC.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10549-023-06911-5.