This paper provides a review and reanalysis of data relating to gastrointestinal (GI) uptake of uranium in humans. Estimates of GI uptake of uranium in adult humans have been derived from results of three controlled experimental studies involving short-term oral intake of an elevated quantity of uranium in fluids, from a controlled balance study conducted in a metabolic research ward of a hospital, and from a variety of environmental studies in which urinary uranium could be related to total intake or total excretion of this element in the same population. For controlled studies, uptake values range from less than 0.1% to about 6% for individual subjects, with central values for different studies falling in the range 1-2.4%. Environmental studies yield central estimates in the range 0.3-3.2%. Expressed as a percentage of total intake of uranium in food and fluids, average GI uptake of uranium in adult humans appears to be about 1-1.5%. Limited intake and excretion data for environmentally exposed human subjects > or = 5 y of age do not reveal important differences with age in uranium uptake, but more definitive information is needed for children. More information is also needed to determine whether fractional uptake of uranium increases with decreasing levels of intake and whether uptake from drinking water is substantially higher than uptake from food. Data for laboratory animals indicate that fractional uptake of uranium depends strongly on the chemical form ingested and the length of time since the last intake of food.