Background : The prognostic nutritional index (PNI) is a useful parameter that indicates the immunonutritional status of patients with malignant tumors. In this retrospective study, we aimed to investigate the value of PNI to predict the outcome of gastrointestinal stromal tumors (GISTs). Material and methods : This study enrolled 431 GIST patients who underwent curative resection from January 2000 to December 2012. A receiver operating characteristic (ROC) curve analysis was used to identify the cutoff value of PNI, neutrophil to lymphocyte ratio (NLR) and platelet to lymphocyte ratio (PLR). Survival curves were produced using the Kaplan-Meier method and were compared using a log-rank test. The Cox proportional hazards model was used to identify independent prognostic factors. Results : Of the 431 patients, 209 (48.5%) were male and 222 (51.5%) were female. The median age was 56 (range 20-80 years old). The PNI cutoff value was 47.45, with a sensitivity of 61.1 % and a specificity of 69.9 %. Compared to the PNI-low group (PNI < 47.45), the PNI-high group (PNI ≥47.45) had a significantly longer recurrence-free survival (RFS) (5-year RFS rate 89.9% versus 70.8%, p<0.001). Patients with higher PNI (p<0.001), lower NLR (p<0.001) and lower PLR (p=0.002) had significant better prognosis. PNI was found to be an independent prognostic factor of RFS (hazard ratio [HR] =1.967, 95% confidence interval [95% CI]: 1.243-3.114, p=0.004). Conclusions : PNI is a simple and useful marker that can predict the prognosis of GIST.