Several reports have suggested an increased prevalence of osteopenia and osteoporosis in HIV-infected individuals. Vitamin D deficiency may be a risk factor for osteoporosis and bone fractures. We aimed to determine the prevalence of vitamin D insufficiency in an outpatient HIV clinic in Boston. We collected serum levels of 25-OH vitamin D and evaluated calcium and vitamin D intake in adult HIV-positive outpatients during the winter and spring of 2005. Fifty-seven subjects were enrolled. The prevalence of moderate (< or = 20 and>10 ng/ml) and severe (< or =10 ng/ml) 25-OH vitamin D deficiency was 36.8% and 10.5%, respectively. Lower vitamin D intake was significantly associated with severe 25-OH vitamin D deficiency (p=0.01). Lactose intolerance tended to be associated with severe vitamin D deficiency (p=0.08). Antiretroviral use and low daily calcium intake were significantly associated with elevated parathyroid hormone levels (p=0.01 and 0.03, respectively). Vitamin D deficiency was frequent in ambulatory HIV-positive patients. HIV-infected individuals living in areas with low exposure to ultraviolet light during winter may benefit from vitamin D supplementation.