Mean age at diagnosis and measurement of breast volume were 59.6 years (standard deviation, SD: 5.9) and 916.8 cm³ (SD: 464.7) for women with screen-detected cancer and 58.9 years (SD: 5.9) and 836.9 cm³ (SD: 446.5) for women with interval cancer (p<0.05 for both) (Figure 1). Mean breast volume increased while fibroglandular volume decreased by age at diagnosis (Figure 2AB). Further, mean breast volume was consistently lower, while mean fibroglandular volume was higher among women with interval cancer compared to those with screen-detected cancer regardless of age at diagnosis (Figure 2AB). Odds of interval cancer decreased with increasing breast volume (Figure 3). Using the first quartile of breast volume as reference, OR for interval cancer was 0.69 (95%CI 0.57-0.84) for quartile 2, 0.55 (95%CI 0.45-0.68) for quartile 3, and 0.45 (95%CI 0.36-0.55) for quartile 4 (p<0.001 for all). Odds of interval cancer increased with increasing fibroglandular volume (OR 1.01, 95%CI 1.01-1.02, p<0.001). The main study limitations were homogenous screening population and lack of information on confounders including breast cancer risk factors, i.e. body mass index and use of hormone replacement therapy. The use of data was approved by the Regional Committee for Medical and Health Research Ethics (#510838).