Fifty-six patients who had been diagnosed diabetic prior to the age of 30 were evaluated to determine the C-peptide (CP) secretory response to a glucose load. These individuals were classified clinically as having insulin dependent (IDDM = 18); non-insulin dependent (NIDDM = 19) and insulin requiring diabetes (IRDM = 19). Insulin dependent diabetics had lower basal CP levels (0.44 +/- (SE) 0.1 ng/ml) which were not stimulated by hyperglycaemia (0.55 +/- 0.13 ng/ml) as compared to controls (basal CP = 1.6 +/- 0.2 and peak 6.2 +/- 0.8 ng/ml). Non-insulin dependent diabetics and insulin requiring diabetics could be divided broadly into two groups - one, a set of patients with low basal CP levels (NIDDM = 0.63 +/- 0.09 ng/ml) (IRDM = 0.38 +/- 0.08 ng/ml) and a blunted response to a glucose load (peak response NIDDM = 0.83 +/- 0.05 ng/ml, IRDM = 0.59 +/- 0.12 ng/ml) and a second group who had CP reserve evident in both fasting (NIDDM = 1.6 +/- 0.2 ng/ml; IRDM = 2.1 +/- 0.6) and post-glucose levels (Peak Response NIDDM = 4.6 +/- 0.4 ng/ml; IRDM = 3.0 +/- 0.6 ng/ml). Growth Hormone (GH) and cortisol levels were found to be high in patients with IDDM and IRDM with no insulin reserve and these did not suppress during the oral Glucose Tolerance Test. NIDDM patients with no insulin reserve had normal GH and high cortisol levels. It is emphasized from this study that insulin sensitivity is as important as the insulin secretory status in determining the presenting features of diabetes mellitus in the young.