We hypothesized that blood flow impacts on arteriolar wall-to-lumen ratio and that vasodilatory capacity is negatively related to arteriolar wall-to-lumen ratio in the human retinal vascular bed.The study cohort comprised 141 non-diabetic untreated male patients with (n=52) or without (n=89) arterial hypertension but without evidence for cardiovascular disease. Retinal capillary blood flow (RCF) before and after exposure to flicker light and to infusion of nitric oxide (NO) synthase inhibitor N-monomethyl-l-arginine (L-NMMA), and parameters of retinal arteriolar morphology, e.g. wall-to-lumen ratio, were assessed non-invasively and in vivo by scanning laser Doppler flowmetry.The study cohort was grouped according to the median RCF into two groups. Patients with RCF above the median revealed lower wall-to-lumen ratio (0.30±0.1 vs 0.34±0.1 (−), P adjusted=0.023) compared to patients with RCF equal or below the median. In addition, RCF was negatively related to wall-to-lumen ratio independently of cardiovascular risk factors (ß=−0.224, P=0.026). In parallel, the decrease of RCF to L-NMMA infusion was greater in patients with RCF above the median compared to the counter group (−8.95±11 vs. 0.35±15 (%), P adjusted <0.001). The increase in RCF to flicker light exposure was negatively related to wall-to-lumen ratio in hypertensive but not in normotensive or all patients (r=−0.292, P=0.047, r=−0.035, P=0.746 and r=−0.126, P=0.144, respectively).In the retinal circulation blood flow impacts on arteriolar wall-to-lumen ratio. Basal NO activity might modulate blood flow and arteriolar morphological changes. In hypertensive, but not in normotensive patients, the vasodilatory capacity is negatively related to arteriolar wall-to-lumen ratio in the human retinal vascular bed.