Objective To explore the value of SUV in patients with ascites of unknown cause.Methods A total of 55 patients (24 males,31 females,average age 54.8 years,age range:23-82 years) with ascites of unknown cause who underwent 18F-FDG PET/CT scan were retrospectively enrolled in this study.SUVmax and SUVmean of ascites and liver were measured and their ratios (T/NT) were calculated and compared by two-sample t test.All patients were followed up and had final diagnoses.Sensitivity,specifici ty,accuracy,positive predictive value and negative predictive value were calculated respectively.The diagnostic efficiency was compared among 18F-FDG PET/CT tumor localization alone,ascites radioactivity and ascites cytology examination by x2 test or Fisher exact test.Results Besides the high metabolized primary lesions and/or metastasis,the liver and spleen swamped by ascites with high radioactivity were visualized on MIP images.The SUVmax and SUVmean of malignant ascites were 1.78±0.65 and 1.37±0.38 respectively,which were higher than those of benign ascites (1.11±0.36 and 0.72±0.22; t=4.13,6.82,both P＜0.05).T/NT of malignant ascites was higher than that of benign ascites (SUVmax:0.64±0.20 vs 0.48±0.12,t =3.27,SUVmeax:0.68±0.17 vs 0.38±0.10,t =7.21 ; both P＜0.05).The diagnostic sensitivity,specificity and accuracy of SUV in ascites were 75.0％ (27/36),94.7％ (18/19) and 81.8％ (45/55),respectively.The sensitivity and accuracy of SUV were higher than those of cytological examination (44.4％ (16/36) and 63.6％ (35/55) ; x2 =6.98,4.58,both P＜0.05).The specificity of SUV was higher than that of tumor localization by 18F-FDG PET/CT (63.2％,12/19; x2 =5.70,P＜0.05).Conclusion Significantly higher SUVmax and SUV in malignant ascites than benign ascites were noted,which might play an adjuvant role in patients with ascites of unknown cause.