Background: Cultural, environmental, and diet-related factors were postulated to lead to a high prevalence of vitamin D deficiency in Kabul's socioeconomically deprived children under 5 years of age. We investigated the prevalence of plasma 25-hydroxyvitamin D (25-OHD) deficiency in such a group. Methods: Children between 6 months and up to 5 years of age were randomly sampled in the Chindawal area of Kabul in January 2005. Plasma samples were frozen to below -20 degrees C and 25(OH)D3 concentrations estimated by high-pressure liquid chromatography. Results: For all 107 children tested, the median plasma 25(OH)D concentration was 5 ng/mL with a range of 2-24 ng/mL, 73% had concentrations of < 8 ng/mL, 13 other samples were not analyzed due to insufficiency of plasma, staff, or technical problems. About 35 others approached either did not give consent or blood could not be obtained. Conclusions: This study was conducted in a high-risk population at a peak season for vitamin D deficiency. We conclude that this population of children living in Kabul to be at great risk of developing vitamin D deficiency, rickets, and other possible immunological effects of deficiency.