Sixty-two patients (34 men and 28 women) aged 19-64 years, half of whom had familial hypercholesterolemia, treated initially for 18 weeks with lovastatin alone were randomly allocated either to lovastatin (L) and cholestyramine (16 g/day) or lovastatin and guar gum (L + GG 20 g/day) treatment for 18 additional weeks to compare the hypocholesterolemic effects of these two combination therapies. The patients were selected for this study from a larger study of patients (n = 120) with severe hypercholesterolemia [serum total cholesterol (serum Chol) 6.5-16.3 mM before treatment], and only those patients in whom serum Chol after lovastatin alone (dose 80 mg/day) remained greater than or equal to 5.2 mM were eligible for evaluation of combination therapies. Serum Chol decreased from 10.6 +/- 1.6 to 5.9 +/- 1.3 mM (mean +/- SD) (p less than 0.001) and low-density lipoprotein cholesterol (LDL Chol) from 8.5 +/- 1.8 to 4.1 +/- 1 mM (p less than 0.001) in patients treated with L + GG (values before the beginning of lovastatin and at the end of the combination therapy). The respective changes were from 10.9 +/- 2.2 to 5.5 +/- 1.2 mM (p less than 0.001) and from 8.7 +/- 2.3 to 3.5 +/- 1.2 mM (p less than 0.001) in patients treated with lovastatin and cholestyramine (L + C). At the end of the study, both serum Chol (p less than 0.005) and LDL Chol (p less than 0.01) were significantly lower with L + C than with L + GG.(ABSTRACT TRUNCATED AT 250 WORDS)