Background: The recommended first-line treatment for advanced, ER+/HER2 negative breast cancer is a CDK 4/6 inhibitor in combination with an endocrine backbone. This study investigated the use of palbociclib, as first- or second-line therapy for advanced breast cancer patients in a real-world setting. Material and Methods: This retrospective, population-based study included all Danish, advanced breast cancer patients with ER+/HER2 negative disease who initiated first- or second-line treatment with palbociclib from January 1^st, 2017, until December 31^st, 2020. The primary outcomes were PFS and OS. Results: The study included 1054 advanced breast cancer patients with a mean age of 66.8 years. Median OS was 51.7 months (95% CI, 44.9–54.6) for all patients in the first-line setting (n = 728) and median PFS was 24.3 months (95% CI, 21.7–27.8). Patients treated in second line (n = 326) had a median OS of 32.5 months (95% CI, 29.9–35.9) and a median PFS of 13.6 months (95% CI, 11.5–15.7). In first-line setting, the PFS and OS were significantly different for endocrine sensitive patients treated with AI (aromatase inhibitor) (n = 423) vs. fulvestrant (n = 158) as endocrine backbone to palbociclib (median PFS AI 31.3 months vs fulvestrant 19.9 months, p = 0.002 and median OS AI 56.9 months vs. fulvestrant 43.6 months, p = 0.001). In endocrine resistant patients (n = 145), no statistically significant difference in PFS was shown (median PFS AI 21.5 months vs. fulvestrant 12.0 months, p = 0.09), whereas OS was significantly different (median OS AI 43.5 months vs. fulvestrant 28.8 months, p = 0.02). Conclusion: In this real-world study, treatment with palbociclib combination therapy met the standards of efficacy set by the phase III trials, PALOMA-2 and PALOMA-3, and the standards set by real-world studies in other countries. The study showed significantly different outcomes in terms of PFS and OS in endocrine sensitive patients comparing AI vs. fulvestrant as endocrine backbone to palbociclib as first-line therapy.