A frequently used index of beta-cell function from the OGTT is the insulinogenic index, IGI. However, there is still some controversy about its validity. In a group of 145 women with different degrees of glucose tolerance, we compared IGI to the corresponding index with C-peptide, DeltaCP(30)/DeltaG(30), which better describes beta-cell function. We also validated both indices with measurements of beta-cell function derived from IVGTT. IGI strongly correlated (R = 0.82, P < 0.0001) with DeltaCP(30)/DeltaG(30). Both IGI and DeltaCP(30)/DeltaG(30) correlated significantly with the corresponding index from IVGTT, though IGI correlation was stronger (IGI: R = 0.67, P < 0.0001; DeltaCP(30)/DeltaG(30): R = 0.56, P < 0.0001). Also indices derived from areas under the curve of insulin, glucose and C-peptide were analyzed. Finally, we compared IGI to similar indices with samples at 60, 90 and 120 min, more often available than that at 30 min. We conclude that IGI is an acceptable index of beta-cell function, as also mirrored by DeltaCP(30)/DeltaG(30). However, the weaker correlation of the C-peptide index with the more accurate index from the IVGTT suggests that it should be used with caution. The index at 60 min can be used as surrogate of IGI, but not the indices at 90 and 120 min.