BACKGROUND Low vitamin D serum levels represent an additional risk factor for end-stage renal disease (ESRD)–associated bone disorders. The aim of this study was to evaluate the prevalence of vitamin D deficiency in both the summer and winter seasons, as well as to define the associated risk factors, and to establish the link between secondary hyperparathyroidism and circulating 25-hydroxy vitamin D (25 [OH]-D) serum levels. METHODS Seventy-eight hemodialysis (HD)–requiring ESRD outpatients living in Buenos Aires, Argentina, age 57.2 ± 2.07 years (21–86 years) were evaluated at the end of summer (SUM) and the end of winter (WIN). Then 25 (OH)-D, serum calcium (Ca), intact parathormone, ionized Ca (Ca11), phosphate, alkaline phosphatase, creatinine, C-reactive protein, and serum albumin were evaluated. The participants were evaluated according to the 25 (OH)-D levels following KDOQI guidelines. The participants were assigned to 2 groups according to their Karnofsky scores for functional capacity (FC): FC 1–2 (normal physical capacity full-time to only part-time) and FC 3–4 (limited capacity only for self care to needs permanent assistance). RESULTS Seasonal variations in 25 (OH)-D (WIN 18.1 ± 1.15 ng/mL vs. SUM 29.2 ± 1.35 ng/mL, p