To determine whether preoperative levels of glycated hemoglobin (GHb) are lower in patients with functioning insulinoma and, if so, whether a distinct separation of GHb values from those in control subjects might serve for diagnosis.We examined preoperative GHb in consecutive patients (who had this measurement done) with surgically confirmed insulinoma for the period 1983 (when the current method became available for routine use) through 1996. Hemoglobin A(1) (HbA(1)) was measured by the Isolab Glyc-Affin Test System (normal range, 4 to 7%). We studied 64 patients with insulinoma (40 women and 24 men; median age, 47.5 years; age range, 21 to 79) and 38 control subjects (25 women and 13 men; median age, 42.5 years; age range, 20 to 83) considered not to have a hypoglycemic disorder on the basis of normal results of a supervised 72-hour fast.HbA(1) was significantly lower in patients with insulinoma (median, 4.7%; range, 2.7 to 6.9%) than in control subjects (median, 5.3%; range, 4.1 to 6.4%) (P<0.001, two-tailed rank sum test). Among 15 patients with insulinoma treated with diazoxide preoperatively, HbA(1) was higher (median, 4.8%; range, 4.2 to 6.9%) than in patients not treated with diazoxide (median, 4.6%; range, 2.7 to 5.7%), although the difference was not statistically significant (P = 0.08).Because of considerable overlap in HbA(1) values, no GHb value was diagnostic for insulinoma; however, 16 of 64 patients (25%) with insulinoma had HbA(1) values below the lowest value (4.1%) in control subjects. Thus, HbA(1) values less than 4.1% in patients with possible insulinoma are strongly indicative of that disorder.