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Background. Studies have suggested that mammographic density and pattern are affected by hormone replacement therapy (HRT) and may influence breast diagnosis. Because 40% of breast cancers diagnosed at our center are mammographically detected while still clinically occult, mammographic sensitivity is crucial. For this reason we studied the effect of HRT on mammographic density. Methods. During a period of 18 months we studied consecutive women older than 54 years attending for breast screening. We recorded HRT use and dosing regimes. A breast density score (BDS) was developed and applied to all mammograms. Results. Mammograms of 148 HRT users were compared with those of 158 nonusers. HRT users had a significantly higher mean density score (4.7 versus 3.4; p < 0.001). Only 11% of non‐HRT users had high scores compare with 37% among HRT users (p < 0.001). The significant difference remained when women were stratified by age. Duration of HRT (longer or shorter than 5 years) did not affect density scores. Conclusion. HRT is associated with a significant increase in breast density. In turn, density and mammographic sensitivity are related. The possibility that increased breast density will hamper mammographic diagnosis of clinically occult cancers is worrisome.