Familial hypertriglyceridemia or hyperlipoproteinemia type I was detected in three siblings aged 6, 11, and 14 of an otherwise normal Turkish family of 10 members. Initial values ranged from 1780 to 3750 mg/100 ml triglycerides in the milky white serum; cholesterol was normal. Lipoprotein pattern on agarose and acrylamide gel revealed a heavy band of chylomicrons and missing HDL; post-heparin lipolytic activity was decreased to about 30% of normal. Chylomicronemia could be induced by a fat-rich (50% of total calories) diet, but not by carbohydrates. On a low fat diet (5%) during hospitalization chylomicrons disappeared, and triglycerides decreased to about 450 mg/100 ml. Phenocopies of hypertriglyceridemia could be excluded. All three patients were the only members of the family who were small, below the third percentile. Their bone age was retarded from 18 to 30 months. There was no indication for an endocrine cause of the growth retardation: four different stimulation tests revealed normal growth hormone response, thyroid and adrenal functions were not impaired; sexual development was normal. Increased glucose assimilation was observed during intravenous and oral glucose load. Peak serum insulin response was above normal during stimulation tests. The possible etiologic role of hypertriglyceridemia in this concomitant growth retardation is discussed.