The presence of multifocality and the aggregate tumor size were retrospectively analysed in a database of 1071 operated breast cancers. Around a quarter of all these cancers involved multiple foci, while a tenth of the total demonstrated more than one invasive focus. Although the multifocal cancers were smaller and more often screen-detected than the unifocal cancers, their aggregate tumor size was larger, and they more frequently displayed casting-type calcifications in the mammogram and HER2 positivity. Lobular histology favoured larger tumor burden. The invasive multifocal cancers were more commonly lymph node-positive than the other tumors. In a subgroup of 584 patients with a median follow-up time of 5 years, the larger size of the invasive tumor, the presence of LVI or lymph node involvement, HER2 positivity and triple negativity were associated with a poorer RFS and OS, while the outcome of screen-detected tumors was superior to that of non-screen-detected or interval cancers. A large tumor size, lymph node positivity and HER2 positive or triple negative phenotypes were independent determinants of a poorer survival rate.