Abstract Aims In left-sided infective endocarditis (IE), a large vegetation >10 mm is associated with higher mortality, yet it is unknown whether surgery during the acute phase opposed to medical therapy is associated with improved survival. We assessed the association between surgery and 6-month mortality as related to vegetation size. Methods and results Patients with definite, left-sided IE (2008–2012) from The International Collaboration on Endocarditis prospective, multinational registry were included. We compared clinical characteristics and 6-month mortality (by Cox regression with inverse propensity of treatment weighting) between patients with vegetation size ≤10 mm vs. >10 mm in maximum length by surgical treatment strategy. A total of 1006 patients with left sided IE were included; 422 with a vegetation size ≤10 mm (median age 66.0 years, 33% women) and 584 (median age 58.4 years, 34% women) patients with a large vegetation >10 mm. Operative risk by STS-IE score was similar between groups. Embolic events occurred in 28.4% vs. 44.3% (P < 0.001), respectively. Patients with a vegetation >10 mm was associated with higher 6-month mortality (25.1% vs. 19.4% for small vegetation, P = 0.035). However, after propensity adjustment, the association with higher mortality persisted only in patients with a large vegetation >10 mm vs. ≤10 mm: hazard ratio (HR) 1.55 (1.27–1.90); but only in patients with large vegetation managed medically [HR 1.86 (1.48–2.34)] rather than surgically [HR 1.01 (0.69–1.49)]. Conclusion Left-sided IE with vegetation size >10 mm was associated with an increased mortality at 6 months in this observational study but was dependent on treatment strategy. For patients with large vegetation undergoing surgical treatment, survival was similar to patients with smaller vegetation size.