Guar gum, a dietary fiber known to improve glucose tolerance, was fed to rats with established diabetes to determine its effect on renal enlargement and microalbuminuria. Diabetic rats were fed a modified AIN-76A (basal) diet for 4 wk, at which time half the rats continued to receive the same basal diet (DB-BA group) and half were switched to a 5% guar gum diet (DB-GG group). Nondiabetic rats fed the basal diet served as controls (NRL group). After 8 additional weeks the animals were killed. Glycated hemoglobin, a measure of long-term blood glucose control, was 14.4% in the DB-BA group and 12.4% in the DB-GG group, a statistically significant difference (P < 0.05). Kidney weight of the DB-BA group (3.51 g) was significantly greater than that of the DB-GG group (2.76 g) (P < 0.05). Eight weeks after induction of diabetes, 24-h urinary albumin excretion was highest in the DB-BA group and lowest in the NRL group; excretion in the DB-GG group (4 wk of guar feeding) was intermediate. However, by 12 wk no differences in albumin excretion among the groups were apparent. These results suggest that guar gum may be useful for slowing the progression of diabetic nephropathy and that guar gum deserves further study in this regard.