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 defi ciency in both the summer and winter seasons, as well as to defi ne 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 Ϯ 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 (Ca ϩϩ ), 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). CONCLUSIONS: Fifty percent of the HD-requiring ESRD patients showed 25 (OH)-D insuffi ciency/defi ciency. This fi nding was more frequent in winter. Women and older patients were at higher risk. Those individuals displaying optimal 25 (OH)-D levels showed better health indicators for HD-requiring ESRD patients. FC assessed through the Karnofsky score was a useful clinical parameter able to identify patients at risk of 25 (OH)-D defi ciency. In our population, renal disease-associated secondary hyperparathyroidism did not correlate with vitamin D defi ciency.