Twenty-nine hypercholesterolaemic patients, treated for one year with gemfibrozil but being still hypercholesterolaemic (serum total cholesterol greater than or equal to 6.25 mmol/l) were included in a double-blind trial to evaluate the hypocholesterolaemic effects of gemfibrozil-guar gum combination (GE + GU) vs. gemfibrozil-placebo combination (GE + PL) using a cross-over study design. The patients were treated with gemfibrozil on a constant dosage (range 900-1200 mg/day) during the entire trial. After a 4-week run-in period on GE + PL treatment the patients were randomly allocated to 2 groups: one received GE + GU 15 g/day, and the other GE + PL for 3 months and after that groups were crossed over. Guar gum and placebo were administered as granules taken 3 times a day during meals. Serum total cholesterol was 8.61 +/- 0.17 mmol/l before gemfibrozil therapy, and 7.29 +/- 0.15 mmol/l at the end of the run-in period on GE + PL (P less than 0.01). During the double-blind phase serum total cholesterol values were 6.28 +/- 0.19 mmol/l at the end of the GE + GU treatment period and 7.21 +/- 0.16 mmol/l at the end of the GE + PL treatment period (P less than 0.01). At the end of the GE + GU treatment period serum total cholesterol was 27% lower and LDL-cholesterol 39% lower than before gemfibrozil treatment. A marked improvement (23%) was found in HDL/LDL ratio during GE + GU treatment compared with GE + PL treatment.(ABSTRACT TRUNCATED AT 250 WORDS)