The aim of the study was to retrospectively determine the impact of comparing current mammograms with prior mammograms on risk of misclassification especially for hormone users. Data on mammography screening were retrieved for 1993-2005 from Fyn, Denmark. At first screen, two projections were made; at subsequent screens, one projection for fatty and two projections for mixed/dense breasts. Until June 3, 2002, 2-year-old mammograms were used for comparison, and later 4-year-old mammograms. Prescription drug data were used to identify hormone, hormone therapy (HT), use. False positive risk and interval cancer proportion dependency on age, hormone use, screen number, projection and prior mammogram were tested with logistic regression. Controlled for breast density, current HT-users had a lower risk of a false positive test 0.69 (95%CI 0.55-0.86) and a lower interval cancer proportion 0.66 (95%CI 0.45-0.99) when 4-year-old instead of 2-year-old mammograms were used for comparison. The use of 4-year-old instead of 2-year-old mammograms for comparison lowered the risk of false positive test in never users, but otherwise age of comparison mammogram had no impact on classification of never and past users of HT. The study indicated that misclassification at screening mammography in current users of HT can be reduced considerably, when the screening mammograms are viewed with the mammograms taken 4 years earlier. It should be stressed that these results come from a single clinic, and replication in other observational and/or experimental studies is warranted.