Human immunodeficiency virus (HIV) infected individuals are prone to malnutrition, and deficiencies of some minerals and vitamins. The aim of this study is to evaluate the frequency of vitamin D deficiency and determine the possible risk factors associated with this problem in HIV-infected individuals. This cross-sectional study was performed on 98 adult patients referred to the Emam Khomeini Hospital Complex, Tehran, Iran. The patients' serum vitamin D concentration was determined using radioimmunoassay method. The possible correlations between demographic and clinical data with the level of vitamin D were evaluated. Vitamin D levels less than 35 nmol/l were considered as deficient in this study. Eighty-five (86.7%) of the patients had serum vitamin D deficiency (concentrations less than 35 nmol/l) in this study. Coinfection with hepatitis C virus (HCV) was present in 54 (55.1%) of the patients. Only daily intake of vitamin D (r = 0.304, p = 0.002), duration of sun exposure (r = 0.268, p = 0.009), the level of PTH (r = -0.459, p < 0.001), daily intake of calcium (r = 0.239, p = 0.018) and GFR of more than 90 ml/min (OR = 1.208, CI 95% = 1.080-1.350, p = 0.033) had a correlation with serum vitamin D concentration. Being female (OR = 7.224, CI 95% = 3.640-14.335, p < 0.001), unemployed (OR = 1.627, CI 95%= 1.209-2.190, p < 0.001) and infected with HCV (OR = 1.811, CI 95% = 1.331-2.465, p < 0.001) were related to the severe serum vitamin D deficiency. Vitamin D deficiency is a common problem in Iranian HIV-infected patients and with concern of this vitamin's important role in health issues, early evaluation of its status and providing appropriate nutritional support seems to be important.