Adjuvant treatment of early-stage breast cancer with combined trastuzumab and chemotherapy has become standard in patients with HER2-positive tumors and a diameter of >1 cm or positive lymph nodes. Currently, there are no data directly supporting the use of adjuvant treatment, including trastuzumab, in patients with HER2-positive tumors, a diameter of ≤1 cm and no nodal involvement (pT1a,bpN0M0). However, 6%-10% of these small tumors are HER2 positive, and there is good evidence for an inferior clinical outcome in these patients, with recurrence rates of up to 30% after 5-10 years. Assumed that the relative risk reduction is similar to larger tumors, the absolute benefit should be large enough to consider adjuvant treatment. This review addresses current data regarding the prognosis of small HER2-positive tumors and discusses potential factors to individualize adjuvant treatment in patients with small HER2-positive tumors.