Background: Abundance of tumor infiltrating lymphocytes (TIL) is prognostic in early HER2-positive breast cancer (BC). Response to neoadjuvant therapy (NAT) according to positron emission tomography combined with computed tomography (PET-CT) has been shown to predict pathologic complete response (pCR). There is paucity of data regarding long-term prognostication using PET-CT and the potential value of the combined assessment of both these biomarkers. Methods: PREDIX HER2 (NCT02568839) is a prospective randomized phase 2 trial that compared standard NAT (docetaxel, trastuzumab, pertuzumab) with trastuzumab emtansine, in patients with HER2-positive BC. Overall, 202 patients were included (197 evaluable) and the primary efficacy analysis showed no difference in pCR or event-free survival (EFS) between the two groups (Hatschek, JAMA Oncology 2021). Assessment with fluorine 18–labeled fluorodeoxyglucose PET-CT was performed at baseline and after 2 and 6 treatment cycles, and SUVmax was evaluated as a continuous variable. TILs were assessed at baseline biopsies according to guidelines from the International TIL Working Group (J.H.). The aim of this secondary analysis was to investigate the combined assessment of TIL and PET-CT as an early predictor of response to NAT. Results: Overall, 112 patients underwent baseline PET-CT and 109 after C2, whereas 173 had baseline TIL. In multivariable analysis, baseline SUVmax did not predict pCR (OR_adj= 1.04, 95% CI 0.97-1.12, p = 0.259) or EFS (HR_adj= 1.07, 95% CI 0.98-1.17, p = 0.117). In contrast, higher SUVmax at C2 predicted lower pCR (OR_adj= 0.65, 95% CI 0.48-0.87, p = 0.005) and worse EFS (HR_adj= 1.18, 95% CI 1.04-1.34, p = 0.01). Baseline TIL > 10% (median cut-off) provided additional prognostic information to clinical parameters (stage and hormone receptor expression) and C2 SUVmax (LR-Δχ² = 7.19, p = 0.007; OR_adj= 3.52, 95% CI 1.37 – 9.06, p = 0.009). 75% of patients with high TIL and C2 SUVmax < 2.49 achieved pCR, compared with 13.8% of those with low TIL and high C2 SUVmax and 39.1%-41.3% for the intermediate groups (p = 0.001). Conclusions: SUVmax after two cycles of NAT for HER2-positive BC is an independent predictor of both short- and long-term outcomes. A combined assessment with TIL may facilitate early selection of good responders for de-escalation and poor responders for alternative treatment strategies. Clinical trial information: NCT02568839.