Background The perforation of upper gastrointestinal tract, primarily caused by peptic ulcer or cancer, is afflicted by a notoriously high mortality rate. The selection of appropriate risk assessments and therapeutic alternatives becomes important when addressing the risk for morbidity and mortality. We aimed to evaluate the optimal treatment and the post-treatment complications for this condition.Methods We retrospectively analyzed 50 patients with intraperitoneal free air due to perforated stomach or duodenum who were consecutively treated at a single institution between 2010 and 2019.Results All patients received initial inpatient treatment that was categorized as either surgery (n = 43, 86%) or non-surgery (n = 7, 14%). The non-surgically cured patients were significantly younger and had no or localized peritonitis, no ascites, lower C-reactive protein (CRP) levels, and shorter hospital stay than the surgery patients. Of seven non-surgery patients, two patients were converted to surgery for worsening symptoms. One of them, who was elderly and had a longer perforation-to-treatment time, stayed at the hospital more than 2 months after surgery with CD Grade Ⅱ. Evaluation of postoperative complications using the Clavien-Dindo classification showed that the patients with Grade Ⅱ–Ⅴ (n=21) were significantly older and had higher heart rates, poorer physical status, and longer perforation-to-surgery than those with Grade 0–Ⅰ (n=24). Preoperative CRP, prothrombin time, and lactate were significantly higher, and hemoglobin was significantly lower in the patients with Grade Ⅱ–Ⅴ. They had significantly longer operation times and found acute renal failure more frequently. Postoperative findings showed a significantly more prolonged period of antibiotic administration, fasting, and hospital stay. The postoperative blood examinations of them showed that minor changes were observed in WBC and neutrophil, and neutrophil and CRP were significantly higher after surgery. The multivariable analyses identified elevated lactate as an independent risk factor for postoperative complications. The postoperative outcomes in patients with perforated gastric cancer depended on the stage and whether a curative resection could be performed.Conclusions Consideration should be given to the indications of non-surgery in elderly patients as well as the delay of treatment and postoperative outcomes of patients with elevated lactate preoperatively.