The absorption of simultaneously administered equimolar doses of 14C vitamin D3 and 3H 25 hydroxyvitamin D3 (25-OH-D3) has been studied in controls and patients with a variety of gastrointestinal disorders. As assessed from peak radioactivity in the serum and from faecal excretion of radioactivity, malabsorption of both vitamin D3 and 25-OH-D3 occurred in patients with steatorrhoea. Malabsorption of vitamin D3 was greater than 25-OH-D3. The magnitude of malabsorption of these compounds was related to the severity of the steatorrhoea but was moderate enough to suggest that replacement therapy in patients with intestinal malabsorption should be accomplished with relatively small doses of vitamin D. The more potent vitamin D metabolites are probably unnecessary in this situation.