Purpose: To measure changes in breast density in a screening population. Method: Unprocessed mammograms were collected for 8,268 women (6034 and 2234 women with two and three sequential screening rounds respectively) with normal breasts (routine recall), from the OPTIMAM image database. The volumetric breast density (VBD), fibroglandular volume (FGV) and breast volume (BV) were determined and the changes between screening rounds calculated. Linear regression determined if the rate of change in these breast density measures varied significantly with age at initial screen. The women were split into four quartiles according to VBD in both screening rounds, and any changes in the quartile allocation of each woman determined. The VBD for these women was compared to our previously published data for women with screen detected and interval cancers. Results: Averaged over all women, the percentage change in VBD, FGV and BV, over 6 years was -11.2% (95%CI: - 12.2% to -10.2%), -5.3% (95%CI: -6.1% to -4.5%) and 11.5% (95%CI: 10.4% to 12.6%) respectively. The percentage change per month, of VBD, FGV and BD decreased significantly with age (p<0.0001). The percentage change in FGV was more strongly associated with the FGV at initial screen than age. For the least and most dense quartiles (who would be of interest for risk stratification), the majority of women did not change quartile between screening rounds. VBD was higher for women who developed interval cancers. Conclusions: The average VBD decreased by 11% over six years. The majority (~80%) of women do not change quartile of VBD in six years.