AIM: This paper evaluates the effect of the adjunct of the hydrosoluble fiber glucomannan to a Step-One-Diet in 40 plasma hypercholesterolemic children, during a randomized controlled trial, to reduce plasma cholesterol. METHODS: All the subjects recruited underwent an 8-week run in diet period; a Step-One-Diet was prescribed. After that, they were randomly allocated to one of two groups: Step-One-Diet only (control), and Step-One-Diet plus glucomannan in gelatine capsules. After another 8 weeks of treatment, the results were compared within and between the two groups. RESULTS: Glucomannan treated group showed decreased values in plasma total cholesterol (TC) and low density lipoprotein cholesterol (LDL-C) vs. control group after 8 weeks of treatment. The percentage decrease showed a statistically significant difference between sex groups. Decreases were observed in favor of female vs. male children in TC (24% vs. 9%) and LDL-C (30% vs. 9%). CONCLUSIONS: These results suggest that glucomannan may represent a rationale adjunct to diet therapy in primary prevention in high risk hypercholesterolemic children.