INTRODUCTION: There has been increasing interest in the potential benefit of vitamin D in improving breast cancer outcome. Preclinical studies suggest that vitamin D enhances chemotherapy-induced cell death. We investigated the impact of serum vitamin D levels during neoadjuvant chemotherapy (NAC) on the rates of achieving pathologic complete response (pCR) after breast cancer NAC. PATIENTS AND METHODS: Patients from 1 of 2 Iowa registries who had serum vitamin D level measured before or during NAC were included. French patients enrolled onto a previous study of the impact of NAC on vitamin D and bone metabolism were also eligible for this study. Vitamin D deficiency was defined as < 20 ng/mL. pCR was defined as no residual invasive disease in breast and lymph nodes. A Firth penalized logistic regression multivariable model was used. RESULTS: The study included 144 women. There was no difference between the French and Iowan cohorts with regard to age at diagnosis (P = .20), clinical stage (P = .22), receptor status (P = .32), and pCR rate (P = .34). French women had lower body mass index (mean 24.8 vs. 28.8, P < .01) and lower vitamin D levels (mean 21.5 vs. 27.5, P < .01) compared to Iowan patients. In multivariable analysis, after adjusting for the effects of cohort, clinical stage, and receptor status, vitamin D deficiency increased the odds of not attaining pCR by 2.68 times (95% confidence interval, 1.12-6.41, P = .03). CONCLUSION: Low serum vitamin D levels were associated with not attaining a pCR. Prospective trials could elucidate if maintaining vitamin D levels during NAC, a highly modifiable variable, may be utilized to improve cancer outcomes.