Objective To investigate the correlation between nocturnal penile tumescence (NPT) test results and metabolic syndrome (MS).Methods In this prospective study,75 cases were enrolled,including 25 healthy volunteers (group A),25 erectile dysfunction (ED) patients with normal NPT results (group B),and 25 ED patients with abnormal NPT results (group C).At the time of recruitment,all the participants had serum total testosterone higher than 9.9 nmol/L,and were free of MS.At baseline,there was no significant differences in waistline,body mass index,fasting plasma glucose (FPG),mean arterial pressure (MAP),triglyceride,high density lipoprotein (HDL),cholesterol,and serum total testosterone among the three groups (P＞0.05).And there was no significant difference in international index of erectile function 5 score between group B and C.A follow-up survey was conducted three years after baseline to measure the above metabolic indicators and to compare the incidence of MS among the three groups.Results In the three-year follow-up survey,2 participants in group A were lost to follow-up,compared to 3 participants in group B and 5 participants in group C.In the three-year follow-up survey,there was no significant difference in all the indicators between group A and B; between group A and C,significant differences were observed in MAP,FPG,HDL,and cholesterol (P＜0.05); between group B and C,significant differences were observed in MAP and HDL (P＜0.05).In the three-year follow-up survey,2 participants in group A developed MS,compared to 3 MS patients in group B and 7 MS patients in group C.There was a significant difference in the incidence of MS among the three groups (P＜0.05).Conclusions To some extent,NPT reflects and predicts the development and progress of MS.Therefore,NPT can be used as an early indicator of MS.