A possible mechanism of the formation of microspherocytosis in unsplenectomized patients with hereditary spherocytosis (HS) was investigated in relation to lipid metabolism in plasma and in red cells of these patients. Plasma lipids (total cholesterol, free cholesterol, high density lipoprotein cholesterol, free fatty acids, and phospholipids) were markedly reduced in unsplenectomized HS patients with microspherocytosis. Red cell membrane lipids (free cholesterol and phospholipids such as phosphatidyl ethanolamine, sphingomyelin, phosphatidyl choline, and lysophosphatidyl choline) were also decreased in these unsplenectomized HS patients. After splenectomy, microspherocytosis disappeared concomitant to substantial normalization of plasma and red cell membrane lipids. These observations suggest that plasma lipid decrement in the unsplenectomized HS patients is at least one of the causative factors in pathogenesis of the formation of microspherocytosis. So-called "splenic conditioning" may not imply a physical "loss" of membrane by fragmentation or pitting of the once-formed HS red cells in the peripheral circulation. Instead, the decreased plasma lipids in the presence of the spleen may affect the de novo synthesis of red cell membrane lipids, resulting in the formation of microspherocytosis.