A 13CO2 starch breath test was developed to study starch digestion in 13 healthy volunteers and 16 patients with pancreatic disease. As starch is hydrolyzed to glucose that is subsequently oxidized to CO2, the excretion of 13CO2 after starch intake, compared with the excretion of 13CO2 after glucose intake, should be a measure of starch hydrolysis. In normal subjects both the hourly 13CO2 excretion rate and the cumulative percentage of 13CO2 excretion after starch ingestion were significantly different from those after glucose ingestion. Comparable differences were found with the patient group. In addition, the 13CO2 excretion after starch intake in normal subjects was markedly different from that of patients with pancreatic disease. Even after correction for endocrine pancreatic insufficiency by calculating the ratio of 13CO2 excretion after starch intake over 13CO2 excretion after glucose intake, the difference between the control group and the patient group was statistically significant. This study indicates that starch hydrolysis is the rate-limiting step in starch digestion. It also confirms recent findings suggesting that starch digestion is impaired in some patients with pancreatic disease. The 13CO2 starch breath test may prove to be useful in evaluating the digestibility of various starch preparations in physiologic and pathological conditions.