Palbociclib is a highly selective, reversible, oral inhibitor of cyclin-dependent kinases 4 and 6 that is approved to treat hormone receptor-positive/human epidermal growth factor receptor 2-negative advanced breast cancer. An open-label, single-arm, Japanese phase 2 study was conducted to investigate the efficacy and safety of palbociclib plus letrozole as first-line treatment in 42 postmenopausal patients with estrogen receptor-positive/human epidermal growth factor receptor 2-negative advanced breast cancer. The probability of progression-free survival originally reported at 1 year was 75.0% (90% confidence interval, 61.3-84.4), but median progression-free survival was not attained at the primary analysis. In this report, updated efficacy and safety results with a longer follow-up period are presented. The median duration of treatment in the updated analysis was 33.0 months (range, 1.8-49.2). The probability of progression-free survival at 1 year was 75.6% (90% confidence interval, 62.4-84.7). Median progression-free survival was 35.7 months (95% confidence interval, 21.7-46.7). Objective response rate and disease control rate were 47.6% (95% confidence interval, 32.0-63.6) and 85.7% (95% confidence interval, 71.5-94.6), respectively. Common treatment-related adverse events (all grades; grade 3/4) were neutropenia (100%; 93%), leukopenia (83%; 60%), and stomatitis (76%; 0%). Treatment-related febrile neutropenia was reported in one patient. In general, no clinically meaningful deterioration in health-related quality of life was observed. Palbociclib plus letrozole remained effective and tolerable in Japanese postmenopausal patients with estrogen receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer in this updated analysis.