The efficacy of soybean protein treatment of stable type II hyperlipoproteinaemia was evaluated in 57 patients assigned to the following protocols: (i) substitution of animal protein with soybean protein (19 subjects) and (ii) addition of soybean protein to a standard low-lipid diet (38 subjects). After 16 weeks of treatment, plasma cholesterol was reduced by 29.5% in the first group, and by 29.9% in the second; the difference was not significant. Similarly the reduction in LDL cholesterol was not significantly different between the 2 groups (39% in the first group and 36% in the second). Plasma triglycerides fell by 11.8% and 18.2%, respectively. HDL cholesterol was not modified to any significant extent by soybean protein regimens. These results provide that the addition of soybean protein to a standard low-lipid diet is effective in inducing a significant cholesterol decrease in patients with type II hyperlipoproteinaemia.