The prognostic nutritional index (PNI) is an immune-nutritional index simply provided by a blood test. We retrospectively compared the postoperative outcomes of patients with lumbar disc herniation divided into two groups according to the PNI. Seventy-three patients who underwent surgery at our hospital were included in the study. All patients had herniation between one of the L3/4, L4/5, or L5/S intervertebral discs and underwent one posterior lumbar interbody fusion. These patients were divided into two groups: patients with a PNI of <50 (poorly nourished (PN) group) and patients with a PNI of ≥50 (well-nourished (WN) group). Evaluation items included patient background characteristics, operative time, blood loss, postoperative complications, and length of hospital stay. The results showed that the body mass index was significantly higher in the WN group than in the PN group (p=0.0221). The rates of collagen disease, steroid use, and postoperative complications were significantly higher (p=0.0475, p=0.0073, and p=0.0211, respectively) and the length of hospital stay was significantly longer (p=0.021) in the PN group than in the WN group. In conclusion, this study indicates that postoperative complications and the length of hospital stay are significantly worse in PN patients than in WN patients.