Background/Aim: The blood pressure, the most influencing factor in cardiovascular disease in end-stage renal failure patients, follows a seasonal variation during the year. Since vitamin D<sub>3</sub> is known to be related to sun exposure, we wanted to evaluate the putative participation of the vitamin D<sub>3</sub> metabolism in blood pressure modifications. Methods: We studied 22 stable hemodialysis patients (11 females and 11 males, mean age ± SD 56 ± 1 year) who had been continuously treated in our dialysis unit for more than 1 year between 1994 and 1997 and did not receive pulse vitamin D<sub>3</sub> treatment. Supine systolic and diastolic blood pressures were measured before every dialysis session (>12,000 measurements) and the intact parathormone (iPTH), 25-hydroxyvitamin D<sub>3</sub> [25(OH)D<sub>3</sub>], and 1,25-dihydroxyvitamin D<sub>3</sub> [1,25(OH)<sub>2</sub>D<sub>3</sub>] levels every 3 months (>300 determinations). The mean values of blood pressure per season and per patient were taken for analysis using a 4-year longitudinal study design. Results: The blood pressure varied during the years studied following a seasonal trend. It was highest during autumn and tended to decrease during spring and warmer months. Systolic as well as diastolic blood pressures were significantly correlated with the 25(OH)D<sub>3</sub> levels (p = 0.0291 and p = 0.0327, respectively). No correlation was observed between blood pressure and 1,25(OH)<sub>2</sub>D<sub>3</sub> or iPTH levels. Conclusions: There is a link between blood pressure and 25(OH)D<sub>3</sub> level. This interrelation is not secondary to a iPTH modulation. Although it cannot be excluded that vitamin D<sub>3</sub> and blood pressure vary following a third factor with seasonal variations, since vitamin D<sub>3</sub> varies during the year, mainly following sun exposure, we suggest that vitamin D<sub>3</sub> is one of the factors participating in the seasonal variation of the blood pressure. Other factors known to control the blood pressure and particularly the extracellular volume overload may also participate.