This study was undertaken to test the hypothesis that the diabetes susceptibility gene on chromosome 20_q12 responsible for maturity-onset diabetes of the young (MODY) in a large kindred, the RW family, results in characteristic alterations in the dose-response relationships between plasma glucose concentration and insulin secretion rate (ISR) that differentiate this form of MODY from MODY in subjects with glucokinase mutations. Ten marker-positive subjects and six matched nondiabetic marker-negative subjects from the RW family received graded intravenous glucose infusions on two occasions separated by a 42-h continuous intravenous glucose infusion designed to prime the β-cell to secrete more insulin in response to glucose. ISR was derived by deconvolution of peripheral C-peptide levels. Basal glucose and insulin levels were similar in marker-negative and marker-positive groups (5.3 ± 0.2 vs. 5.0 ± 0.2 mmol/l, P > 0.2, and 86.1 ± 3.9 vs. 63.7 ± 12.1 pmol/l, P > 0.1, respectively). However, the marker-positive subjects had defective insulin secretory responses to an increase in plasma glucose concentrations. Thus, as the glucose concentration was raised above 7 mmol/l, the slope of the curve relating glucose and ISR was significantly blunted in the marker-positive subjects (13 ± 4 vs. 68 ± 8 pmol · min⁻¹ · mmol⁻¹ · 1, P < 0.0001). The reduced insulin secretory responses in the marker-positive subjects were most evident at higher plasma glucose concentrations >7 mmol/1, and differences between the two groups were not significant at lower glucose levels between 5 and 7 mmol/1. After a 42-h glucose infusion, the amount of insulin secreted over the glucose concentration range 5–9 mmol/1 increased by 54 ± 16% in the markernegative subjects. This priming effect of glucose on insulin secretion was not seen in 9 of the 10 markerpositive subjects. In contrast, previous results in MODY subjects with glucokinase mutations showed persistence of the glucose-priming effect on ISR and continued increases, although subnormal, of ISR as plasma glucose concentration rises from 7–12 mmol/1. In conclusion, subjects from the RW family who have inherited the at-risk allele of the MODY1 gene appear to have a characteristic pattern of altered insulin secretory responses to glucose. These alterations are present before the onset of hyperglycemia, suggesting a unique mechanism of (β-cell dysfunction different from the defect in MODY subjects with glucokinase mutations.