Fourteen male subjects with hypercholesterolemia received daily supplementation with granulated guar gum or placebo, 15 g/day, during 12 wk in a double-blind, cross-over trial. A statistically significant reduction in serum total cholesterol (7.27 +/- 0.24 versus 8.23 +/- 0.26 mmol/l, mean +/- SEM, p less than 0.01) which was mainly due to a reduction in low-density lipoprotein cholesterol concentration (4.70 +/- 0.19 versus 5.32 +/- 0.23 mmol/l, p less than 0.05) was observed after 6 wk on guar gum as compared with placebo. Between 6 and 12 wk on guar gum the serum cholesterol and low-density lipoprotein cholesterol levels increased in most subjects, and after 12 wk the difference from placebo was no longer statistically significant. Serum high-density lipoprotein cholesterol levels were unaffected by guar gum. Serum and lipoprotein triglycerides showed no significant changes during the study, and the body weight of the subjects remained unchanged. Serum calcium, magnesium, phosphate, and iron levels, and urinary calcium excretion were not affected by guar gum supplementation. No severe side effects were observed, necessitating reduction of the dose or stopping the treatment. It is concluded that the hypocholesterolemic effect of guar gum seems to decrease during prolonged dietary supplementation. Further controlled studies are needed before the dose response and the long-term effects of guar gum in hypercholesterolemia can be evaluated.