This work was designed to investigate the site of oxalate hyperabsorption in malabsorption syndromes. 1) Urinary oxalate excretion was measured in 27 patients with ileal resection (IR) and steatorrhea. Mean urinary oxalate excretion was high in 13 patients with IR and intact colon and in 9 subjects with IR and right hemicolectomy (90.2 +/- 11.9 and 108 +/- 18.6 mg per 24 hours; mean +/- S.E.M.), whereas it was normal in 5 patients with IR and ileostomy (21.9 +/- 4.4 mg per 24 hours). Steatorrhea was similar in the three groups. 2) On one patient of the last group in whom the colon had not been removed initially but excluded closure of the ileostomy resulted in the development of frank hyperoxaluria. 3) Intracolonic perfusion of calcium (1.93 g per day) abolished or greatly reduced the hyperoxaluria in 3 patients. These results indicate that the colon is the major site of oxalate hyperabsorption, and the right colon is not necessary for the development of hyperoxaluria in malabsorption syndromes.