Our observation that hypoglycemia, often self-diagnosed by our patients, was seldom confirmed led the authors to establish norms for the glucose tolerance test. We first obtained values for 650 patients who were entirely free from symptoms before and during testing. The median nadir in this group was 64 mg/dl. Ten percent of the patients had plasma glucose nadirs of 47 mg/dl or below and 2.5% had values of 39 mg/dl or less. Utilizing these values in combination with clinical criteria, we confirmed hypoglycemia after glucose load in 16 (median nadir 39.5 mg/dl) of 118 patients presenting with this diagnosis, and only 5 of the 16 were hypoglycemic after their usual meals. The other 102 patients, whose many complaints were unrelated to measured plasma glucose levels, had nadirs similar to those of the control group. Placebo tests performed on 14 nonhypoglycemic patients provoked symptoms (recorded by the patients themselves) and they considered indicative of hypoglycemia. Some accepted other diagnoses after we demonstrated that their symptoms occurred when they were normoglycemic. Since nadirs of hypoglycemics and control subjects overlap, we conclude that accurate diagnosis of hypoglycemia requires that symptoms develop concurrently with low blood sugar and that they are absent at other times. Low plasma glucose must be considered only one of the criteria in diagnosing functional hypoglycemia along with a relationship between food intake, timing of symptoms, correlation of symptoms and low glucose levels, and reproducibility of test results.