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Background It is well known that mammographic screening reduces breast cancer mortality. One possible explanation for this effect is that screening makes it possible to detect smaller breast cancers with fewer involved nodes, but another hypothesis is that some screening-detected tumors are in a pathologically and biologically different phase of evolution from those that are detected clinically. The aim of the present study was to compare the biological, pathological and clinical characteristics of symptomatic vs. asymptomatic breast cancers. Patients and methods: The study considers a series of 1916 consecutive patients who underwent surgery for stage I and II infiltrating breast cancer at Verona hospitals after having undergone ultrasound and mammography (at least one of which was positive). They were divided into two groups on the basis of why they decided to undergo the imaging examinations: group A refers to the 1247 patients with a palpable lump, and group B to the 616 who were asymptomatic. Results The patients in group A were older, and had larger tumors and a higher percentage of positive nodes than those in group B; they also had significantly higher grade tumors, higher Ki-67 levels, and a higher percentage of ER and PgR negative and c-erbB-2 positive tumors (all of the P-values were significant). A logistic regression analysis adjusted for tumor diameter and age showed a reduction in the significance of each of the considered variables, but all of them remained significantly associated with the modality of diagnosis except ER, PgR and c-erbB-2. Conclusions Our results suggest that asymptomatic tumors are biologically different from their clinically presenting counterparts, thus confirming the hypothesis that progression towards greater malignancy may occur during the natural history of breast cancer.