Background: Residual and significant post-infarction left ventricular (LV) dysfunction, despite technically successful percutaneous coronary intervention (PCI) for ST-elevation myocardial infarction (STEMI), remains an important clinical issue. In preclinical models low dose insulin-like growth factor 1 (IGF1) has potent cytoprotective and positive cardiac remodelling effects. We studied the safety and efficacy of immediate post PCI low dose intracoronary IGF1 infusion in STEMI patients. Methods: Using a double-blind, placebo controlled, multi-dose study design, we randomized 47 STEMI patients with significantly reduced (≤ 40%) LV ejection fraction (LVEF) after successful PCI to single intracoronary infusion of placebo (n=15), 1.5ng IGF1 (n=16) or 15ng IGF1 (n=16). All received optimal medical therapy. Safety endpoints were freedom from hypoglycaemia, hypotension or significant arrhythmias within 1 hour of therapy. The primary efficacy endpoint was LVEF and secondary endpoints were LV volumes, mass, stroke volume, and infarct size at 2 months follow up, all assessed by MRI. Treatment effects were estimated by analysis of covariance adjusted for baseline (24hrs) outcome. Results: No significant differences in safety endpoints occurred between treatment groups out to 30 days (chi squared test, p-value = 0.77).There were no statistically significant differences in baseline (24 hrs post STEMI) clinical characteristics or LVEF among groups. LVEF at 2 months, compared to baseline, increased in all groups with no statistically significant differences related to treatment assignment. However, compared with placebo or 1.5ng IGF1, treatment with 15ng IGF1 was associated with a significant improvement in indexed LV end-diastolic volume (p=0.018), LV mass (p=0.004) and stroke volume (p=0.016). Late gadolinium enhancement (±SD) at 2 months was lower in 15ng IGF1 (34.5±29.6g) compared to placebo (49.1±19.3g) or 1.5ng IGF1 (47.4±22.4g) treated patients, though the result was not statistically significant (p = 0.095). Conclusion: In this pilot trial, low dose IGF1, given after optimal mechanical reperfusion in STEMI, is safe but does not improve LVEF. However, there is a signal for a dose dependent benefit on post MI remodeling that may warrant further study. Despite timely reperfusion by primary PCI (PPCI) a significant cohort of patients develop adverse left ventricular remodelling with clinical sequelae such as arrhythmia and heart failure.Therapeutic approaches to avert such remodeling, including a variety of cell therapy and ischemia- reperfusion-injury mitigation trials have achieved modest success 2.;3. Thus, there remains a significant opportunity for novel therapies in this field.