15 outpatients with type II hyperlipoproteinemia (7 with type IIa and 8 with type IIb) were treated with soybean. This diet induced a significant decrease of plasma cholesterol levels from 314 +/- 24 to 267 +/- 30 mg/100 ml. Triglycerides levels remained unchanged. LDL-cholesterol decreased significantly from 241 +/- 26 to 194 +/- 32 mg/100 ml, whereas HDL-cholesterol increased only slightly and not significantly from 38 +/- 7 to 40 +/- 8 mg/100 ml. In our efforts to reduce hyperlipidemia the very low cost of these vegetable proteins is a further advantage. The hypocholesterolemic mechanism of soybean diet is unknown.