<b>Objectives:</b> This study aimed to characterize <i>CCNE1</i> copy number (CN) in relation to other molecular characteristics of tumors, along with patient outcomes, to help guide future clinical trial design and potential novel targeted therapeutics development for these aggressive molecular subtypes of high-grade serous ovarian (HGSOC) and endometrial cancers (EMCA). <b>Methods:</b> Tumor samples were analyzed using Next-Gen sequencing (NGS) of the DNA (NextSeq, 592 genes or NovaSeq, whole-exome sequencing) and RNA (NovaSeq, whole-transcriptome sequencing) and immunohistochemistry (IHC) (Caris Life Sciences, Phoenix, AZ). PD-L1 IHC used SP-142 (cut-off ≥1%). Microsatellite instability (MSI) was tested by fragment analysis, IHC, and NGS. Tumor mutation burden (TMB) was measured by totaling somatic mutations per tumor (high ≥10 mutations/MB). Relative abundance of immune-cell infiltrates was calculated by MCPCounter. <i>CCNE1</i> amplification (<i>CCNE1</i><sup>Amp</sup>) was defined as CN ≥6 and calculated by NGS/WES. Real-world overall survival (OS) was obtained from insurance claims data, and Kaplan-Meier estimates were calculated for molecularly defined patient cohorts. Statistical significance was determined using Chi-square and Wilcoxon rank-sum test, and p-values adjusted for multiple comparisons (q) to be <0.05. <b>Results:</b> <i>CCNE1</i> was amplified in 608/6306 (9.6%) of HGSOC, 214/2098 (10.2%) of serous EMCA, 191/749 (15.3%) of uterine carcinosarcomas (UCS), and 17/2053 (0.80%) of endometrioid EMCA. <i>TP53</i> mutations positively correlated with <i>CCNE1</i><sup>Amp</sup>. <i>CCNE1</i><sup>Amp</sup> HGSOC had fewer mutations in <i>BRCA1/2</i>, <i>ARID1A</i>, and <i>NF1</i> but increased CN amplification in other genes (Table 1). <i>PTEN</i> mutations were negatively correlated with <i>CCNE1</i><sup>Amp</sup> in UCS and endometrioid EMCA. There was no difference in immuno-oncology biomarkers (TMB, dMMR/MSI, PD-L1). However, increased <i>CCNE1</i> levels were associated with decreased fibroblasts in HGSOC and serous EMCA, decreased B cells in serous EMCA, and decreased CD3+ T cells in UCS. In HGSOC, <i>CCNE1</i><sup>Amp</sup> was associated with increased expression of cell-cycle checkpoint regulators, Fanconi anemia, homologous recombination, mismatch repair, and single-strand break repair pathway genes, but decreased expression of these genes in endometrioid EMCA. In tumors with >1 specimen serially submitted, there was a significant increase in <i>CCNE1</i> CN over time in 29% (49/169) of HGSOC and 23% of EMCA (36/158). There was no difference in <i>CCNE1</i> CN in tumors harboring <i>BRCA</i> reversion mutations (<i>n</i>=8) compared to wild-type (<i>n</i>=5110). <i>CCNE1</i><sup>Amp</sup> correlated with mRNA expression in HGSOC and EMCA. <i>CCNE1</i><sup>Amp</sup> was associated with worse OS after carboplatin-based treatment in HGSOC as well as in serous EMCA and UCS. <b>Conclusions:</b> <i>CCNE1</i><sup>Amp</sup> in HGSOC and EMCA is less common than previously identified. Both HGSOC and endometrioid EMCAs have distinct molecular characteristics of <i>CCNE1</i><sup>Amp</sup> tumors. <i>CCNE1</i> CN may increase with the emergence of treatment resistance. Finally, <i>CCNE1</i><sup>Amp</sup> tumors appear overall non-immunogenic.