It is thought that the risk of atypical hyperplasia (AH) increases with age, particularly among postmenopausal women. Three hypotheses were investigated to try to explain this phenomena: use of hormone replacement therapy (HRT), increased breast cancer screening and improvements in radiological quality. Data were collected from the Bouches du Rhône breast cancer screening programme database and from the pathological registry of all women operated on for breast diseases in the district. The AH incidence rate was studied using a Poisson regression analysis. The change in the profile of breast diseases was explored through studying changes in the proportion of AH among benign lesions and malignant diseases. The AH incidence rate significantly increased over time (13.6% per year). The proportion of AH among the benign diseases increased with time and was significantly higher for HRT users (Odds Ratio (OR)=2.05; 95% Confidence Interval (CI): 1.43-2.93). While AH decreased with age among HRT non-users, it increased among users as a proportion of both benign and malignant lesions. The AH incidence rate significantly increased among pre- and postmenopausal women. Our study suggests that this increase is partly explained by the incidental discovery of these lesions by mammography and partly by a real increase of the disease among HRT users.