IntroductionSurgery for acute empyema is associated with postoperative complications and relapse. Establishing a predictor for postoperative complications may improve prognosis.ObjectivesTo demonstrate undernutrition as a predictor of complications after surgery for acute empyema.MethodsWe retrospectively analyzed 52 consecutive patients who underwent surgery for acute empyema from 2004 to 2019 and validated the correlation of undernutrition with serum albumin level, patient characteristics, hospital stay, and postoperative complications.ResultsThe median preoperative serum albumin level was 2.4 g/dL (range: 1.1‐3.4). The levels in all patients were lower than the standard value (3.5 g/dL). Patients were divided into two groups based on the median serum albumin levels: the low serum albumin level group (group L, n = 28) and the high serum albumin level group (group H, n = 24). Group L patients were significantly older (64.5 vs 52.9 years, P = .002), had lower median body mass index (21.0 vs 24.2, P = .008), and significantly had Streptococcus anginosus group as the causative bacteria (50% vs 21%, P = .044). Their hospitalization duration was significantly longer (28.1 vs 14.8 days, P < .001), and postoperative complications were significant or tended to be more frequent (all incidence; 11 (39%) vs 2 (8%), P = .012, respiratory‐related; 7 (25%) vs 1 (4%), P = .056) in group L. Further analyses revealed that other undernutrition indicators also correlated with postoperative complications.ConclusionsPreoperative serum albumin level is a valid predictor of complications after surgery for acute empyema. Preoperative nutrition management for empyema patients may reduce the occurrence of postoperative complications.