Plasma concentrations of 5,000 daltons (5 kDa) immunoreactive gastric inhibitory polypeptide (IR-GIP) were measured before and up to 16 hours after the start of low-dose insulin treatment in newly diagnosed ketotic type I (insulin-dependent) diabetics. Nine patients were non-fasting. Before insulin treatment mean IR-GIP was 31 +/- 6 pmol/l (range 9-65 pmol/l). Four patients had IR-GIP concentrations in the normal fasting range (10-25 pmol/l), and nine patients had concentrations below 35 pmol/l. The remaining patients had IR-GIP concentrations in the normal postprandial range. A meal eaten after the start of insulin treatment caused an increase in IR-GIP in all patients. All patients had beta-cell function as estimated by plasma C-peptide. Individual changes in C-peptide were significantly correlated to changes in blood glucose both after the meal (r = 0.80, p less than 0.01) and during insulin treatment (r = 0.85 +/- 0.04). No correlation could be found between IR-GIP and blood glucose, C-peptide or insulin concentrations. Newly diagnosed ketotic type I diabetics have IR-GIP concentrations within the normal postprandial level. Hypoinsulinaemia, hyperglycaemia, and hyperketonaemia do not by themselves increase 5 kDa IR-GIP markedly above normal fasting levels.