OBJECTIVE : Wholegrain foods vary in the extent of processing. We investigated whether wholegrain particle size in bread influences postprandial glycemia in type 2 diabetes. METHODS : Postprandial glycemia (incremental area under the blood glucose curve [iAUC]) was measured after consumption of three breads made with roller-milled wholegrain flour and added grains and a fourth made with stone-ground flour. All flours and grains were 100% wholegrain wheat. Breads were nutrient matched. RESULTS : Fifteen adults (64 ± 10 years, HbA1c 58 ± 13 mmol/mol) completed the study. iAUC for the three breads made with roller-milled flour ranged from 376 to 641 mmol-1min-1, inverse linear trend for grain particle size P = 0.039. The iAUC for stone-ground wholegrain bread (503) was smaller than predicted from mean particle size. CONCLUSIONS : Wholegrain structural integrity in bread is a determinant of glycemic response. These findings have implications for dietary advice and the definition of the term "'wholegrain."