The effects were tested of two doses of soy protein on lipid, lipoprotein and homocysteine concentrations. After 4–24 weeks of consuming a lipid-lowering diet, 130 men and women with low-density lipoprotein (LDL) cholesterol concentrations ≥ 4 mmol/l were studied in a parallel group trial in which four interventions were assigned randomly. A beverage containing 30 g isolated soy protein (ISP) and 10 g cotyledon fibre or 50 g ISP and 16.6 g cotyledon fibre, or equivalent doses of casein and cellulose, was consumed daily for 16 weeks. When the two groups that consumed ISP were compared with the two groups that consumed casein, the differences in the net changes from baseline to week 16 in the concentrations of LDL cholesterol and plasma total homocysteine were –0.26 mmol/l [95% confidence interval (CI) –0.4 to –0.09 mmol/l] and –0.8 μmol/l (95% CI –1.4 to –0.2 μmol/l), respectively. The effect of the ISP dose was not significant. There were no significant differences between the two ISP and the two casein groups in changes in lipoprotein(a), high-density lipoprotein (HDL) cholesterol, or triacylglycerol concentrations. The authors concluded that adding 30–50 g soy protein per day to a lipid-lowering diet significantly reduced LDL cholesterol concentrations without increasing lipoprotein(a) concentrations. Plasma total homocysteine concentrations also decreased, suggesting a novel, possibly anti-atherosclerotic effect.