The effect of guar mini-tablets (5 g t.i.d.) on carbohydrate and lipid metabolism of outpatients with overt diabetes mellitus with glycosuria (is greater than 5 g/24 h) was determined in an open-controlled, randomized, multicenter, crossover study. A 4-wk pretreatment period was followed by a 6-wk treatment period. The treatment period consisted of a 2-wk guar period (treatment period II), which was followed by the wash-out period. The other half of the patients received treatment in the reverse order. Out of 93 patient records, 79 (41 sulfonylurea [SU] and 38 insulin-treated) were suitable for statistical analysis. No relevant weight-reducing effect of guar could be found in both 2-wk treatment periods. At the end of treatment period II, the lowering of the 1-h postprandial values of blood glucose (SU 12%, insulin 10%), cholesterol (SU and insulin 25%) was significant after 2-wk of guar treatment compared with the wash-out period. No clinically relevant changes in the safety laboratory parameters were observed during guar treatment. Side effects were observed in 40 of the 93 patients included in the trial. Treatment had to be discontinued in 11% of the patients due to gastrointestinal side effects. On the basis of our results,guar treatment in combination with sulfonylurea and insulin can be recommended for the improvement of carbohydrate and lipid metabolism.