Background: In 2018 breast cancer was the 2nd most diagnosed malignancy worldwide and leading cause of cancer-associated death in women in Mexico. The most common subtype is hormone receptor positive/HER2 negative (HR+/HER2-). Adding a cyclin dependent kinase 4/6 inhibitor (CDK4/6i) to the hormonal treatment (HT) in metastatic breast cancer (MBC) increases the progression-free (PFS) and overall survival (OS) of these patients. Objective: Describe the use of CDK4/6i in patients with MBC with RH+/HER2- in a third level hospital in Mexico City. Methods: Review of medical records of patients with MBC RH+/HER- in treatment with CDK4/6i from July 2016 to May 2019. Clinical and pathological variables were collected. The PFS was calculated by Kaplan-Meier and the survival distribution was compared using Log-rank test; for the association of variables, Chi Square or Student T was used according to the distribution. Results: A total of 44 patients were collected, 42 received palbociclib and 2 ribociclib. The median age was 52.2 years (22-79). Within the characteristics of the pathology, 34 patients had infiltrating ductal carcinoma, 8 infiltrating lobular and 2 mixed histology. Regarding the metastasis pattern, 29 presented visceral and 15 non-visceral; bone was in 64.6% of the patients, hepatic 41.5%, lymph node 33.8%, pulmonary 21.5%, other sites 18.5%, central nervous system 3.1%. At the cut-off date, the median PFS in the 44 patients was 10 months (1-60); 18 patients (40.91%) used a CDK4/6i in first line and 26 (59.09%) in subsequent lines. The PFS in the patients with CDK4/6i as first line has not been reached, unlike those in subsequent with 13.5 months (1-27), Odds Ratio (OR) for first line versus subsequent is 0.16 (95% CI 0.043-0.647, p=0.014). The OR in PFS with HR, proliferation index, clinical stage or ECOG was not significant. Associating metastasis pattern and PFS, patients with lung metastases are at greater risk: OR:9.40 (95% CI 1.73-51.10, p=0.006), the other metastatic sites were not significant; 23 patients (52.28%) had any toxicity; the most common was hematologic with 25 events (56.8%); dose reduction of palbociclib was required in 11 women (25%), 7 (63.6%) to 100 mg and 4 (36.3%) to 75 mg and 27 non-hematological events (61.36%) were presented. Conclusions: These results suggest that the first line for MBC RH+/HER2- should include an CDK4/6i in combination with HT. Pulmonary metastases may be associated with a worse prognosis.