The attribute(s) of soluble dietary fibers responsible for cholesterol lowering is currently uncertain. A series of experiments were conducted in which viscosity and fermentability was assessed independently for their effect on plasma and liver cholesterol concentration. Hamsters were divided into four dietary groups and fed diets containing 0.12% cholesterol and 5% fiber as high viscosity hydroxypropyl methylcellulose (HV-HPMC group), low viscosity hydroxypropyl methylcellulose (LV-HPMC group), high viscosity guar gum (HV-GG group) or low viscosity guar gum (LV-GG group). Hydroxypropyl methylcellulose is essentially nonfermentable, whereas guar gum is highly fermentable. Plasma cholesterol concentrations at 3, 6 and 11 wk and liver cholesterol concentrations at 6 and 11 wk were significantly lower in the HV-HPMC group relative to the LV-HPMC group (P < 0.05). Intestinal content viscosities of the LV-HPMC and HV-GG groups were similar; consequently, these two groups were compared to examine the independent effect of fermentation. Plasma and liver cholesterol were significantly lower in the HV-GG group compared with the LV-HPMC group at 6 wk (P < 0.05), but not at 3 or 11 wk. Hepatic sterol synthesis rates were not affected by any of the diets. This study shows that greater viscosity of intestinal contents is strongly associated with cholesterol reduction, but that the contribution of fiber fermentation remains uncertain.