Colesevelam hydrochloride is a novel, potent, non-absorbed lipid-lowering agent previously shown to reduce low density lipoprotein (LDL) cholesterol. To examine the efficacy and safety of coadministration of colesevelam and atorvastatin, administration of these agents alone or in combination was examined in a double-blind study of 94 hypercholesterolemic men and women (baseline LDL cholesterol > or =160 mg/dl). After 4 weeks on the American Heart Association Step I diet, patients were randomized among five groups: placebo; colesevelam 3.8 g/day; atorvastatin 10 mg/day; coadminstered colesevelam 3.8 g/day plus atorvastatin 10 mg/day; or atorvastatin 80 mg/day. Fasting lipids were measured at screening, baseline and 2 and 4 weeks of treatment. LDL cholesterol decreased by 12-53% in all active treatment groups (P<0.01). LDL cholesterol reductions with combination therapy (48%) were statistically superior to colesevelam (12%) or low-dose atorvastatin (38%) alone (P<0.01), but similar to those achieved with atorvastatin 80 mg/day (53%). Total cholesterol decreased 6-39% in all active treatment groups (P<0.05). High density lipoprotein cholesterol increased significantly for all groups including placebo (P<0.05). Triglycerides decreased in patients taking atorvastatin alone (P<0.05), but were unaffected by colesevelam alone or in combination. The frequency of side effects did not differ among groups. At recommended starting doses of each agent, coadministration of colesevelam and atorvastatin was well tolerated, efficacious and produced additive LDL cholesterol reductions comparable to those observed with the maximum atorvastatin dose.