In 12 patients with chronic non-nephrotic renal failure (creatinine clearance 3.5-11 ml/min) and 8 control subjects, plasma triglyceride turnover was studied. The patients showed significantly increased fasting triglyceride concentrations and absolute plasma triglyceride turnover rates, while the fractional turnover rate was significantly decreased. The finding of an elevated triglyceride turnover rate with a near normal fractional removal rate in the patients with the highest creatinine clearance suggests the involvement of an increased triglyceride synthesis rate in these patients. However, most patients showed an impaired removal mechanism as the major cause of the hypertriglyceridemia.