Purpose: To quantify B-1 transmission-field inhomogeneity in breast imaging of normal volunteers at 3T using 3D T-1-weighted spoiled gradient echo and to assess the resulting errors in enhancement ratio (ER) measured in dynamic contrast-enhanced MRI (DCE-MRI) studies of the breast. Materials and Methods: A total of 25 volunteers underwent breast imaging at 3T and the B-1 transmission-fields were mapped. Gel phantoms that simulate pre- and postcontrast breast tissue T-1 were developed. The effects of B-1-field inhomogeneity on ER, as measured using a 3D spoiled gradient echo sequence, were investigated by computer simulation and experiments on gel phantoms. Results: It was observed that by using the patient orientation and MR scanner employed in this study. the B-1 transmission-field field is always reduced toward the volunteer’s right side. The median B-1-field in the right breast is reduced around 40% of the expected B-1-field. For some volunteers the amplitude was reduced by more than 50%. Computer simulation and experiment showed that a reduction in B-1-field decreases ER. This reduction increases with both B-1-field error and contrast agent uptake. Conclusion: B-1 transmission-field inhomogeneity is a critical issue in breast imaging at 3T and causes errors in quantifying ER. These errors would be sufficient to reduce the conspicuity of a malignant lesion and could result in reduced sensitivity for cancer detection.