Hydroxypropylmethylcellulose (HPMC) and methylcellulose (MC) are modified cellulose dietary fibers that generate viscous solutions in the gastrointestinal (GI) tract. This study assessed the effects of high viscosity (HV) HPMC, ultra-HV (UHV) HPMC, and medium viscosity MC on postprandial glucose and insulin responses in overweight and obese men and women (n = 50). After overnight fasts, subjects consumed 5 breakfast meals containing 75 g carbohydrate, each of which contained 1 of the following: 1 g HV-HPMC, 2 g HV-HPMC, 2 g UHV-HPMC, 4 g medium-viscosity MC or control (2 g cellulose). Test sequence was randomized and double-blind, except the MC test, which was last and single-blind (46 subjects completed all 5 tests). Glucose and insulin responses were determined pre-meal and for 120 min postprandially. Median (interquartile limits) peak glucose concentration was lower (P = 0.001) after the meal containing 2.0 g UHV-HPMC (7.1, 6.3-8.2 mmol/L) compared with the control meal (7.7, 6.6-8.7 mmol/L). The control did not differ from the other conditions for peak glucose or for any of the HPMC/MC conditions for glucose incremental areas under the curves (IAUC). Peak insulin was reduced (P < 0.05) for all HPMC/MC conditions compared with control. Insulin IAUC was lower than control (P < 0.001) after meals containing 2 g HV-HPMC, 2 g UHV-HPMC, and 4 g MC. GI symptoms did not differ among treatments. These findings indicate that HV-HPMC (1 and 2 g), UHV-HPMC (2 g), and MC (4 g) consumption reduced postprandial insulin excursions consistent with delayed glucose absorption.