Pantethine (P), the stable disulphate form of pantetheine, major component and precursor of coenzyme A, was evaluated within a double-blind protocol (8 weeks for P or for a corresponding placebo) in 29 patients, 11 with type IIB hyperlipoproteinemia, 15 with type IV, and 3 with an isolated reduction of high density lipoprotein cholesterol (HDL-C) levels. In type IIB patients, P (300 mg t.i.d.) determined a highly significant lowering of plasma total and low density lipoprotein (LDL) associated cholesterol (-13.5% for both parameters). In the same patients, HDL-C levels increased about 10% at the end of treatment. Switching from P to placebo was associated with a rapid return to the baseline cholesterolemia. Both in type IIB and type IV patients, plasma triglyceride levels were reduced around 30%, when P was given as the first treatment; when it was preceded by placebo, reductions were less striking (respectively, -17.8% for type IIB and -13.0% for type IV, at the end of P treatment). HDL-C levels were not increased by P, either in type IV, and in the patients with low HDL cholesterolemia. In type IV, LDL cholesterol levels showed a variable response to P: they tended to increase when below 132 mg/dl, prior to treatment, and to be reduced when above this level. This study provides evidence for a significant hypocholesterolemic effect of P, a natural compound free of overt side effects. It also indicates that P may raise HDL-C levels in type IIB patients, while moderately reducing triglyceridemia.