To determine the effect of lipoprotein lipase deficiency on the size distribution of fat cell populations in human adipose tissues, abdominal and femoral subcutaneous fat tissue biopsies were obtained from seven patients affected by familial hyperchylomicronaemia. These patients were characterized by massive accumulation of chylomicrons in the fasting state due to defective catabolism of plasma triglyceride-rich lipoproteins. They had no post-heparin plasma lipoprotein lipase activity and their fat tissues were deficient in lipoprotein lipase activity. The size distribution of adipocytes examined by scanning electron microscopy were similar to distributions observed in control subjects. Patient fat cell diameters were not statistically different from control fat cells obtained from subjects of similar body mass index. Mature fat cells contributed to 99% of the total fat tissue mass in lipoprotein lipase deficiency. Normal adiposity in lipoprotein lipase deficiency can thus be attributed to mature adipocytes and not to hyperplastic growth of immature fat cells. It is concluded that normal adipose tissue homeostasis is maintained in these patients in spite of the deficiency in lipoprotein lipase activity.