BACKGROUND Interventional treatment improves prognosis in patients with acute coronary syndromes (ACS). However, despite introduction of percutaneous coronary intervention (PCI), the risk of cardiovascular events in patients with multivessel coronary artery disease (MVD) remains significant. AIM To evaluate the risk of complications and the prognostic value of MVD in patients with ACS during 1-year follow-up. METHODS A group of 153 patients with ACS was followed up at a single cardiology unit with round-the-clock PCI capability. Treatment of ACS, the extent of revascularisation, and complications occurring during hospitalisation and 1-year follow-up were analysed. The end points of the study were defined as death from all causes, cardiac death, recurrent ACS and a composite end point (deaths from cardiac causes and recurrent ACS). RESULTS During 1-year follow-up, 11 (7.2%) patients died, including 10 patients with MVD without complete revascularisation. Recurrent ACS occurred in 18 (12%) patients, including 13 patients with MVD without complete revascularisation. Presence of a residual significant coronary stenosis in incompletely revascularised patients with MVD was an important risk factor for all-cause mortality and occurrence of a composite endpoint in comparison to MVD patients who underwent complete revascularisation (p = 0.028 and p = 0.046, respectively) and patients with single-vessel disease (p = 0.006 and p = 0.003, respectively). CONCLUSIONS Incomplete revascularisation during the acute phase of ACS was associated with an increased risk of complications and a significantly increased risk of all-cause mortality and the combined rate of cardiovascular deaths and recurrent ACS. Single-stage PCI of all significant stenoses in MVD patients resulted in better outcomes.