The purpose of this study was to evaluate nonpalpable breast masses identified on mammograms that are sonographically occult.The pathology data base at the Woman's Place for Breast Care of the Woman's Hospital of Texas was searched to identify patients who had undergone biopsy for a nonpalpable breast mass identified on mammograms in which a mass was not visible at sonography.There were 32 of 231 such nonpalpable masses that were sonographically occult. Twenty-eight of the masses were histologically benign; four were malignant.A small percentage of mammographically visible nonpalpable breast masses are sonographically occult; a majority of these masses are benign. However, biopsy of such masses should be considered and the decision to biopsy based on mammographic features and interval change.