Disproportionate elevation [increased proinsulin/insulin (PI/INS) ratio] of PI immunoreactivity is associated with noninsulin-dependent diabetes mellitus (NIDDM). The nature of this abnormality is not known. To address the question of whether genetic factors contribute to hyperproinsulinemia, we measured fasting levels of PI immunoreactivity, intact INS, and C peptide (CP) in 12 pairs of monozygotic twins discordant for NIDDM for a mean (+/- SEM) period of 9 +/- 3 yr. Thirteen age- and body mass index-matched healthy subjects without any family history of NIDDM acted as controls. The nondiabetic twins had levels of fasting INS, CP, PI, PI/CP, and PI/INS similar to those of control subjects. Fasting levels of PI, and PI/CP and PI/INS ratios were significantly 2- to 3-fold elevated in NIDDM twins compared to those in both nondiabetic twins and control subjects. To investigate whether hyperproinsulinemia in these NIDDM patients was due to a differential elevation of intact PI or conversion intermediates, we analyzed PI profiles in NIDDM twins and normal subjects by high pressure liquid chromatography. PI was heterogeneous and consisted mainly of des(31,32)-PI and intact PI in both NIDDM patients and normal subjects, with no major difference in composition between the groups. Small amounts of des(64,65)-PI (0-11%) were measured in some patients and normal subjects. The results suggest that hyperproinsulinemia is not a genetically determined trait per se in NIDDM. Disproportionately elevated PI levels seem to be related to the actual disease process. Further conversion of intact PI and des(31,32)-PI may be equally impaired in NIDDM.