Trastuzumab has revolutionised the treatment of HER2-positive early-stage breast cancer and is now standard of care in combination with chemotherapy for patients with tumours larger than 1 cm. However, 5 years after publication of the landmark trials establishing the efficacy of the drug, the management of small (≤1 cm), HER2-positive tumours remains difficult. Most small breast cancers have a good prognosis and adjuvant chemotherapy is not routinely recommended. However, retrospective data suggest that some small HER2-positive cancers might have a worse clinical outcome than others. This notion raises the key clinical question of whether patients with small HER2-positive cancers should be offered adjuvant trastuzumab and chemotherapy. The pivotal adjuvant trastuzumab trials did not include patients with tumours smaller than 1 cm, but a subset analysis of one trial showed that patients with tumours 1–2 cm in size derived at least as much clinical benefit from 1 year of adjuvant trastuzumab as did the overall cohort. Clinicians face the dilemma of whether the potential reduction in risk of recurrence in this patient group warrants the toxic effects and risks of adjuvant chemotherapy and trastuzumab. In this review, we discuss the evidence for prognosis of small HER2-positive cancers, and for possible benefit from adjuvant trastuzumab. We suggest potential treatment strategies and clinical trial designs to address this important issue. On the basis of present evidence, we recommend that the benefits and risks of adjuvant trastuzumab should be discussed with patients with small, HER2-positive breast cancer.