To clarify the mechanism of hyperinsulinism of hepatic cirrhosis, plasma insulin and C-peptide levels before and after oral glucose loads were measured in 34 patients with cirrhosis, 15 patients with chronic hepatitis, and 25 normal subjects. While plasma immunoreactive insulin (IRI) levels during oral glucose tolerance testing (OGTT) were significantly increased in cirrhotics, plasma immunoreactive C-peptide (CPR) levels were elevated slightly. The C-peptide to insulin ratio throughout OGTT was significantly smaller in cirrhotics than in normal subjects (P less than 0.01). A decreased hepatic insulin degradation rate has been suggested to one of the main causes of hyperinsulinism in hepatic cirrhosis. The ratio of the difference between basal and 30-min CPR values and basal and 30-min OGTT blood glucose values [delta CPR: delta BS(30)'] as well as the delta IRI: delta BS(30') ratio was significantly decreased in cirrhotics (P less than 0.01). These results indicate that insulin secretion in response to a glycemic stimulus is reduced in cirrhotics. Both the ratios of the sums of six IRS and CPR values of OGTT (sigma CPR: sigma IRI) and delta CPR: delta BS(30') and sigma CPR: sigma BS(30') were found in inverse relationship with indocyanine green retention rate in cirrhotics.