Differences in breast density between populations may explain part of the variation in regional breast cancer screening performance. This study aimed to determine whether regional differences in breast density distribution are present in the Dutch screening population. As part of the DENSE trial, mammographic density was measured using a fully-automated volumetric method. The regions in our study were based on the geographic coverage of 14 reading units representing a large part of the Netherlands. General linear models were used. Four hundred eighty-five thousand and twenty-one screening participants with a median age of 60 years were included (2013-2014). The proportion of women with heterogeneously or extremely dense breasts ranged from 32.5 % to 45.7 % between regions. Mean percent dense volume varied between 6.51 % (95 % confidence interval [CI]: 6.46-6.55) and 7.68 % (95 % CI: 7.66-7.71). Age differences could not explain the variation. Socio-economic status (SES) was positively associated with volumetric density in all analyses (low SES: 6.95 % vs. high SES: 7.63 %; p (trend) <0.0001), whereas a potential association between urbanisation and breast density only became apparent after SES adjustment. There appears to be geographic variation in mammographic density in the Netherlands, emphasizing the importance of including breast density as parameter in the evaluation of screening performance. aEuro cent Mammographic density may affect regional breast cancer screening performance. aEuro cent Volumetric breast density varies across screening areas. aEuro cent SES is positively associated with breast density. aEuro cent Implications of volumetric breast density differences need to be studied further.