Purpose: This study aims to measure the impact of HRT use at the time of screening on rates of screen-detected invasive breast cancer (IBC) and ductal carcinoma in situ (DCIS), interval cancers and investigative procedures, within a well-established population-based mammography screening program. Methods: Using South Australian BreastScreen data from 1998 to 2009 pertaining to 819,722 screening episodes, Poisson regression models were undertaken to estimate the incidence risk ratios (IRR) for various screening outcomes at both the first and subsequent screening rounds, among women who had been using HRT in the 6 months prior to screening compared with those who had not. Results: Current HRT use was associated with increased risk of recall for assessment, biopsy procedures, and breast cancer diagnosis among BreastScreen participants. Risk of screen-detected breast cancer was increased at subsequent screening rounds (IRR = 1.30, 95 % confidence interval 1.18-1.34), but not at women's first screening round (1.05, 0.88-1.25). This increased risk applied to IBC (1.35, 1.27-1.45), but not to DCIS (1.04, 0.89-1.23). Interval cancer risk was elevated among HRT users following both the first screen (1.77, 1.33-2.37) and subsequent screening episodes (1.92, 1.72-2.15). Conclusions: Increased risks of recall, biopsy rates, screen-detected, and interval cancers among HRT users have important implications for population-based breast cancer screening programs. Our findings support the concept that HRT use may increase the growth of preexisting cancers. Lack of effect on DCIS could imply different etiology or time frames for DCIS and IBC development or increased transition from preinvasive to invasive disease due to HRT use.