The hyperglycemic clamp is considered to be the gold standard for determining both first and second phase insulin secretion. In order to achieve a reasonable insulin plateau for the second phase, it has become common practice to clamp for 120 or even 180 minutes at 10 mM. It is unknown whether earlier insulin determinations would be sufficient to predict second phase insulin secretion. We therefore reviewed the hyperglycemic clamp data of 58 subjects with different degrees of glucose tolerance to assess whether one or more insulin concentrations determined at earlier time points of the clamp could predict second phase insulin secretion (insulin and C-peptide concentration at 120 minutes). The correlation coefficients between second-phase insulin secretion and plasma insulin or C-peptide at 60 min were 0.95 and 0.96, respectively (both p<0.00005). Averaging plasma insulin or C-peptide over 2 or more adjacent time points did not improve the correlation. In conclusion, a one-hour hyperglycemic clamp can provide the standard measurement of first phase insulin secretion plus a good approximation of second phase insulin secretion.