In a randomized, double-blind, cross-over trial soybean phospholipid and placebo, 18 g daily for 6 wk, were given orally to 20 patients on long-term treatment with standard lipid lowering diets. The effect of this treatment on serum total cholesterol and high-density lipoprotein cholesterol and plasma triglyceride was studied. After 6 wk mean (+/- SE) cholesterol concentration was decreased by 0.54 (+/- 0.19) mmol/liter in phospholipid-treated as compared to placebo-treated patients (p less than 0.02). The decrease in serum cholesterol was significant (p less than 0.02) only in patients assigned to receive phospholipid before placebo. A highly significant increase (p less than 0.001) followed the withdrawal of phospholipid. No effect on triglyceride and high-density lipoprotein cholesterol concentrations was demonstrated.