Background: The addition of H to chemotherapy has improved outcomes in HER2-positive early BC. This approach is associated with (w/) an increased risk (<4%) of congestive heart failure (CHF). At a median f/u of 36 months, the addition of anti-HER2 Rx to dose-dense (every 2 weeks) anthracycline-taxane therapy (Rx) was not associated with excess cardiotoxicity. Here we report the incidence of NYHA Class III/IV CHF in 2 phase II studies with longer follow-up. Methods: We conducted a retrospective review of pts w/ HER2 + early stage BC treated at MSKCC and DF/HCC on two trials: In trial A - pts received dd AC (60/600 mg/m²) x 4 → T (175mg/m²) x 4 (w/ pegfilgrastim) w/ H x 1 year. Trial B differed w/ use of weekly T (80mg/m²) x 12 and the addition of L (1000mg orally daily) x 1 year. In both trials, H was begun after completion of AC and concurrent with T. Left ventricular ejection fraction (LVEF) was prospectively assessed by multi-gated acquisition scan serially throughout Rx. Results: Trial A enrolled 70 pts and Trial B enrolled 95 pts w/ the median age of 46 years (range 27-73 years). Overall, the 5-year distant disease-free survival (DDFS) for trials A and B is 92% (95%Cl; 83-97%) and 89% (95%CI; 81-94%), respectively. The baseline median LVEF was 68% (range 52-81%). In total, 28 of 165 (17%) pts had pre-existing hypertension. Now at a median follow-up of 84 and 57 months respectively, only one (1.4%, 95%CI; 1.36-7.7%) and 4 (4.2%, 95%CI; 4.2-10.4%) pts developed CHF. Since our earlier report, 1 additional CHF event occurred (Trial B) at month 44. Conclusions: Longer follow-up of these 2 studies demonstrate that dd AC → TH with or without L is associated w/ a low risk of CHF. This is consistent w/ the long-term cardiac toxicity reported from the randomized phase III studies of H w/ conventionally scheduled anthracycline-based regimens (with or without taxanes). DDFS outcomes are also encouraging. Clinical trial information: NCT00482391.