We tried to compare the parameters of nocturnal penile tumescence and rigidity (NPTR) testing with erectile function (EF) domain score of International Index of Erectile Function (IIEF), which is used in diagnosis and determining the severity of erectile dysfunction (ED), and to assess the sufficiency of IIEF in the diagnosis of ED. A total of 90 men, mean age 46 years (24-75), presenting with ED to our clinic between January 2001 and March 2003 were included in the trial. All the men answered the standard IIEF (15 questions) forms and was divided into four groups as mild ED, moderate ED, severe ED and no ED according to the EF domain score that is obtained from 1st, 2nd, 3rd, 4th, 5th and 15th questions. Then NPTR testing with the RigiScan Plus monitoring device was performed for two consecutive nights on those men. The distribution of the six parameters of NPTR testing (number of erections, duration of erections, TAU base, RAU base, TAU tip, RAU tip) among the four groups and the correlation with IIEF-EF domain score were evaluated. Additionally, the distribution of the risk factors (diabetes mellitus, hypertension, atherosclerotic heart disease, dyslipidemia and smoking) was analyzed both among the four groups and in each group. According to IIEF-EF domain scores of 90 patients, 16 (18%) had severe ED, 21 (23%) moderate ED, 41 (46%) mild ED and 12 (13%) no ED. There was no statistically significant difference between the risk factors among the men in these groups (P > 0.05). When the IIEF-EF domain scores were compared with parameters of NPTR testing, no statistically significant difference was obtained among ED groups (mild, moderate, severe) (P > 0.05). However, we observed a statistically significant difference between three ED groups and no ED group (P < 0.05). If NPTR testing is considered as a gold standard test, sensitivity, specificity, positive predictive value and negative predictive value of IIEF-EF domain score in ED diagnosis are 100, 17.9, 29.4 and 100% respectively. In conclusion, we did not observe a clinical correlation between IIEF-EF domain scores and NPTR parameters in the whole population; however, we observed that if IIEF-EF domain scores were normal, NPTR parameters were also normal. In other words, we can say that if the initial IIEF-EF domain scores are normal, then we do not have to perform NPTR testing. This could be helpful to make a cost-effective diagnosis.