Maturity-onset diabetes of the young (MODY) is a form of non-insulin-dependent diabetes mellitus (NIDDM) associated with autosomal-dominant inheritance. In the RW pedigree, MODY is associated with polymorphic DNA markers on chromosome 20q. To determine the early abnormalities of insulin action and insulin secretion in MODY, we studied nondiabetic members of the RW pedigree with and without the gene marker. Six nondiabetic marker-negative and 5 nondiabetic marker-positive members of the RW pedigree were studied, as were 4 diabetic marker-positive family members. Unrelated, young, healthy subjects served as comparison groups. Insulin action and insulin secretion were assessed with a frequently sampled intravenous glucose tolerance test. Insulin secretion was further assessed during constant glucose infusion by deconvolution of plasma C-peptide and by pulse analysis. The nondiabetic marker-positive group had normal sensitivity to insulin and unimpaired acute insulin response to intravenous glucose (AIR_glu). However, the nondiabetic marker-positive group had decreased mean plasma C-peptide concentration and reduced absolute amplitude of insulin secretory oscillations during prolonged glucose infusion. These responses to prolonged glucose infusion were similar to those observed in the diabetic group. No alterations of insulin secretion were observed in the nondiabetic marker-negative family members. Deranged and deficient insulin secretion, and not insulin resistance, appears to be the genetic or primary abnormality that characterizes nondiabetic individuals who are predisposed to MODY in the RW pedigree. Prolonged glucose infusion studies may reveal qualitative and quantitative defects in insulin secretion not identified by the AIR_gIu. Use of the AIR_glu may not be able to exclude a primary β-cell defect in the pathogenesis of NIDDM.