Preventive percutaneous coronary intervention (PCI) for non-culprit lesions after primary PCI remains controversial in patients with acute coronary syndrome (ACS). We analyzed whether PCI for non-culprit lesions would be associated with a better long-term prognosis in very elderly (≥85 years) patients. This study included 91 consecutive patients with ACS (mean age, 88.2 ± 3.0, 52 % male). We investigated the association of residual lesions with 1-year mortality. Culprit lesions affected the left anterior descending artery (LAD) in 50 patients, the left circumflex artery (LCx) in 29, and the right coronary artery (RCA) in 31. Residual lesions affected LAD in 20 cases, LCx in 22, and RCA in 21 patients. Residual lesions in LAD were associated with a higher 1-year mortality (p = 0.013), whereas residual lesions in LCx or RCA were not (p = 0.547 and 0.473, respectively). A Cox regression model demonstrated that patients with residual lesions in LAD had an increased risk of 1-year mortality compared with those without residual lesions (hazard ratio, 2.39; 95 % confidence interval, 1.16–4.96; p = 0.019). Therefore, the option to not treat residual lesions in LAD of patients with PCI may be associated with a higher 1-year mortality. Further studies are needed to confirm these findings.