The efficacy and safety of the HMG-CoA-reductase inhibitor pravastatin was assessed in a double-blind, placebo controlled study. Thirty patients (51 y) with hyperlipoproteinaemia Type IIa (N = 22) or IIb (N = 8) received for 16 weeks either pravastatin 5 mg b.d. for 8 weeks followed by 10 mg b.d. for 8 weeks (Group I), or 10 mg b.d. for 8 weeks followed by 20 mg b.d. to 16 weeks (Group II), or placebo (Group III). In Groups I and II, mean serum total cholesterol was reduced by -26% and -22%, respectively; low-density lipoprotein (LDL)-cholesterol decreased by -28% and -27%, apolipoprotein B by -25% and -23%, and apolipoprotein E by -9% (NS) and -16%, respectively. Serum high-density lipoprotein (HDL)-cholesterol was increased by 11% in Group II, and so the total/HDL-cholesterol ratio fell by 33%. Apoprotein A1 and A2 were not significantly changed. No serious clinical and laboratory abnormalities were observed. The data suggest considerable therapeutic efficacy of pravastatin in the treatment of Type II hyperlipoproteinaemia.