The relationship between glucose and insulin levels was examined in a prospective study of 153 pregnant patients without diabetes who underwent a standard 50 gm glucose challenge test. One hundred eighteen women had normal screening results (glucose level less than 140 mg/dl) and 35 had abnormal screening values but a normal oral glucose tolerance test. Abnormal responders had greater insulin levels (149 vs 82 pmol/L, p less than 0.0001), and a higher insulin/glucose index (0.96 vs. 0.72, p less than 0.007). Patients with glucose levels less than 100 mg/dl had significantly lower insulin/glucose indices. Overall, obese patients had significantly greater glucose and insulin measurements than did nonobese women, but there was no difference within normal and abnormal groups. Glucose levels accounted for 52% of the insulin output and 29% of the insulin/glucose index variance. Neither age, parity, nor obesity contributed significantly to insulin levels in the multiple regression model. Therefore the accepted threshold for glucose screening reflects abnormal insulin output and this aberration may be indicative of the primary defect in gestational diabetes.