This study investigated the magnitude of vendor-specific differences in visually assessed PD. On average, it was found that visual consensus PD assessments from Siemens images were 2.3 percentage points higher than Hologic consensus PD assessments taken from a mammogram of the same woman within a 12-month timeframe. Such a small between-vendor bias is unlikely to be clinically significant or alter the course of a woman’s follow-up care. Additionally, such a small difference is unlikely to be a source of bias in multivendor studies assessing PD, particularly as PD is often assessed in the more broad BI-RADS density categories, which typically span a 25%-wide category of PD.22 Furthermore, two percentage points fall within the 5% levels that are consistent with radiologists’ internal rating scales of PD.23,24 Additionally, the upper and lower limits of agreement, capturing how far apart 95% of the measurements on vendor-paired mammograms are, are very narrow ( percentage points around the bias) and suggest that the two vendors’ mammography units may be used interchangeably in assessing breast density. In combination with the very strong correlation () and agreement statistic (), the small bias and narrowness of the upper and lower limits of agreement support the argument that visually assessed PD by radiologists agrees across the two mammography device vendor units and, therefore, that radiologists’ visual assessments of PD may be reliable across different mammography device vendors.