Background: Mammographic screening and increased awareness has led to an increase in the detection of T1 breast tumors that are generally estimated as having low risk of recurrence after locoregional treatment. However, even small tumors can metastasize, which leaves us with the question for the necessity of adjuvant treatment. Therefore, additional prognostic markers are needed to tailor adjuvant systemic treatment for these relatively low-risk patients. The aim of our study was to evaluate the accuracy of the 70-gene MammaPrint signature in T1 breast cancer. Materials and Methods: We selected 964 patients from previously reported studies with pT1 tumors (≤2 cm). Frozen tumor samples were hybridized on the 70-gene signature array at the time of the initial study and classified as having good prognosis or poor prognosis. Results: The median follow-up was 7.1 years (range 0.2-25.2). The 10-year distant metastasis-free (DMFS) and breast cancer specific survival (BCSS) probabilities were 87% (SE 2%) and 91% (SE 2%), respectively, for the good prognosis-signature group (n = 525), and 72% (SE 3%) and 72% (SE 3%), respectively, for the poor prognosis-signature group (n = 439). The signature was an independent prognostic factor for BCSS at 10 years (multivariate hazard ratio [HR] 3.25 [95% confidence interval, CI, 1.92-5.51; P < .001]). Moreover, the 70-gene MammaPrint signature predicted DMFS at 10 years for 139 patients with pT1ab cancers (HR 3.45 [95% CI 1.04-11.50, P = .04]). Conclusions: The 70-gene MammaPrint signature is an independent prognostic factor in patients with pT1 tumors and can help to individualize adjuvant treatment recommendation in this increasing breast cancer population.