Background: Palbociclib (PD-0332991) is an orally bioavailable selective inhibitor of CDK4/6 that prevents DNA synthesis by prohibiting progression of the cell cycle from G1 to S phase. In a randomized phase II trial comparing palbociclib (PD-0332991) plus letrozole (P + L) to letrozole (L) in postmenopausal women with ER(+), HER2(–) advanced breast cancer (ABC) who had not received any prior systemic anticancer therapy for their advanced disease, P + L demonstrated significantly longer progression-free survival (PFS) vs L (26.1 vs 7.5 mo; HR = 0.37, P < .001) and was generally well tolerated, with uncomplicated neutropenia as the most frequent adverse event (Finn et al SABCS 2012). Methods: Based on phase II data, a global, randomized, double-blind, phase III clinical trial was designed to demonstrate that P + L provides superior clinical benefit compared with L + placebo in postmenopausal women with ER(+), HER2(–) ABC who have not received any prior systemic therapy for their advanced disease. The study aims to assess whether P + L improves median PFS over L at HR of at least 0.7. Approximately 450 eligible patients with locoregionally recurrent or metastatic, pathologically confirmed ABC who are candidates to receive L as first-line treatment for their advanced disease will be randomized 2:1 to receive either P (125 mg QD 3 wk on, 1 wk off) + L (2.5 mg QD) or L (2.5 mg QD) + placebo. Patients who received anastrozole or letrozole as part of their (neo)adjuvant regimen are eligible if their disease progressed more than 12 months from completion of adjuvant therapy. Tumor tissue is required for participation. Secondary endpoints include overall survival, objective response, duration of response, clinical benefit, safety and tolerability, and patient-reported outcomes of health-related quality of life and disease- or treatment-related symptoms. Clinical trial information: NCT01740427.