This study aimed to assess serum vitamin D as well as vitamin D receptors (VDR) in men with erectile dysfunction (ED) non-responding to oral sildenafil citrate. The study included 160 men out of 684 presenting to the Andrology, Sexology & STIs Department, Faculty of Medicine, Cairo University after ethical approval and informed consent. In all, 160 men were allocated into; healthy potent men (n=40), men with ED responders for sildenafil citrate (n=60) and men with ED non-responders for sildenafil citrate (n=60). Inclusion criteria for sildenafil citrate non-responders were an inadequate erectile response after at least 4 attempts using the highest tolerated drug dose in accordance with manufacturer’s guidelines with respect to timing relative to meals, use of concomitant medications and adequate sexual stimulation/ arousal. Exclusion criteria were; diabetes mellitus, smoking, hypertension, dyslipidemia, cardiovascular disorders, hepatic or renal failures. They were subjected to; history taking, clinical examination, International Index of Erectile Function (IIEF) questionnaire, estimation of serum vitamin D and vitamin D receptor (VDR) gene expression. The results demonstrated that men with ED non-responding for sildenafil demonstrated significant decrease in IIEF questionnaire scores; mean serum vitamin D as well as VDR expression compared with healthy potent men as well as men with ED responding to sildenafil. Age demonstrated non-significant correlation with IEEF, VDR or serum vitamin D. Serum vitamin D demonstrated significant positive correlation with IIEF and VDR. VDR gene expression demonstrated significant correlation with IIEF and serum vitamin D. It is concluded that men with ED non-responding to sildenafil have significant decrease in serum vitamin D as well as VDR expression.