<p>Background: Vitamin D supports oocyte development and ovarian steroidogenesis; deficiency is linked to preeclampsia and poor oocyte quality. The purpose of the current study was to investigate the relationship between vitamin D levels and oocyte quality in women undergoing ICSI cycles.<br /> Methods: In this case-control study, 252 women were identified from medical records and classified into high-quality (n=126) and low-quality oocyte groups (n=126). Serum vitamin D levels and anti-M&uuml;llerian hormone (AMH) were recorded. Oocyte quality was assessed by an embryologist using predefined criteria for morphology and maturity. Statistical analysis included Mann&ndash;Whitney tests and multivariable logistic regression (p&lt;0.05). ROC analysis was applied to determine the optimal vitamin D cutoff, reporting AUC, sensitivity, and specificity and statistical significance was set at p&lt;0.05.<br /> Results: Patients in the high-quality oocyte group had a higher median serum vitamin D level compared with those in the low-quality group (25.2 <em>vs.</em> 21.9 <em>ng/ml</em>). Logistic regression showed that sufficient vitamin D levels (&ge;18 ng/ml) were associated with increased odds of having high-quality oocytes (OR=1.85, 95%CI: 1.07&ndash;3.22). AMH levels were also positively associated with oocyte quality (OR=1.42, 95%CI: 1.11&ndash;1.82). The ROC analysis yielded an AUC of 0.61 (95%CI: 0.53&ndash;0.68), indicating modest discriminatory ability.<br /> Conclusion: Serum vitamin D levels are associated with oocyte quality in women undergoing ICSI cycles. While higher vitamin D levels are linked to better oocyte quality, no causal inference can be drawn from this case&ndash;control study design, and interventional studies are needed to evaluate whether correction of vitamin D deficiency improves reproductive outcomes.</p>