Background: CDKIs with endocrine therapy (ET) is first-line treatment in HR+/HER2- MBC. Mouse models have shown that CDKIs prevent pRB phosphorylation in the mediobasal hypothalamus, a pathway hyper-activated in diet-induced obesity; and CDKIs lead to fat mass decrease without significant effect on lean mass. We aimed to assess the impact of CDKIs on weight (wt) and BC in pts with HR+/HER2- MBC. Methods: We identified pts with HR+/HER2- MBC who received CDKIs and ET from 2015-2018. To isolate the effect of CDKIs on BC, we identified another cohort of pts who only received ET. Body mass index (BMI), wt, and computed tomography (CT) records were reviewed. BC was analyzed at L3 on CT scans using Tomovision’s SliceOmatic v5.0 and included skeletal muscle area (SMA), skeletal muscle density (SMD), subcutaneous adipose tissue (SAT), visceral adipose tissue (VAT), and muscle adiposity (MA). Total adipose tissue (TAT) was defined as SAT+VAT+MA. BC changes at 3 and 6 months of therapy were evaluated using paired t-tests. Results: There were 107 pts who received CDKI plus ET - 43% were Black, and 41% were Hispanic. CDKIs used were palbociclib (85%), abemaciclib (9%), and ribociclib (6%). ETs used were letrozole (47%), fulvestrant (39%), anastrozole (12%), and exemestane (2%). Median number of prior chemo and ET lines was 0 (range 0-5). 63 pts received ET alone. There was no difference in age (63 vs. 65 years, p = 0.26), BMI (28.80 vs. 28.12kg/m², p = 0.48), and visceral disease (69% vs. 65%, p = 0.64) between CDKI plus ET and ET alone group. At month 3 of CDKI plus ET, there was a significant decrease in wt (-0.30kg, Interquartile range [IQR] -2.55-0.95, p = 0.02), BMI (-0.12kg/m², IQR -1.06-0.46, p = 0.02), SAT (-8.05cm², IQR -32.58-14.74, p = 0.01), and TAT (-8.51cm², IQR -50.42-17.84, p < 0.01), with similar results at month 6. These findings were not seen in pts on ET only at 3 months (wt: 0.00kg, IQR -2.65-2.38, p = 0.98; BMI: 0.00kg/m², IQR -1.07-0.91, p = 0.93; SAT: -2.97cm², IQR -26.10-25.15, p = 0.60; TAT: -0.58cm², IQR -44.39-27.43, p = 0.18), or at 6 months. There were no significant changes in VAT, SMA, SMD, or MA in both groups at 3 or 6 months. In the CDKI plus ET group, baseline wt (74.64 vs. 72.72kg, p = 0.60), BMI (29.21 vs. 27.88kg/m², p = 0.31), and SAT (280.29 vs. 252.75cm², p = 0.31) were not significantly different for those who did or did not develop grade 3/4 toxicities. We obtained similar results when stratifying toxicities into hematological- and GI-related events. Conclusions: CDKIs are associated with decrease in BMI and SAT with no significant effect on VAT, SMA, or SMD. Given the known effect of obesity on breast cancer prognosis, CDKIs may have an additional effect on breast cancer prognosis by modulating body fat. Further studies are required to determine if decrease in SAT is associated with breast cancer outcomes or toxicities in pts on CDKIs.