We performed oral fat loading tests in 10 patients with chronic renal failure (CRF) on hemodialysis (5 children and 5 adults). Fat absorption was measured by hourly determination of serum triglycerides (TG), cholesterol (CHOL), and lipoproteins (LP) after oral administration of a 'milkshake' containing 50 g of fat of dairy origin. 10 age-matched healthy volunteers with normal fasting serum TG levels and 10 subjects with fasting hypertriglyceridemia served as controls. Mean fasting serum TG levels in CRF patients were elevated compared to normal controls (177.6 +/- 14.6 mg/dl, 2.0 +/- 17 mmol/l vs. 91.0 +/- 10.5 mg/dl, 1.03 +/- 12 mmol/l). 6 patients (4 adults, 2 children) had type IV LP patterns and 2 patients (both children) showed type IIb hyperlipidemia. In only 2 patients, 1 child and 1 adult were TG, CHOL and LP electrophoresis all normal. The oral fat loading test in all CRF patients showed delayed appearance of TG and chylomicrons (CHYL) in the bloodstream i.c. impaired or slow absorption of fat from the gut. In contrast to normal and hypertriglyceridemic controls, TG and CHYL levels in CRF did not decrease by 5 h after the oral fat load. This study demonstrates impaired intestinal fat absorption in children and adults with CRF.