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The secondary drug failure is a well known phenomenon in the development of type 2 diabetes mellitus, but an exact definition of this situation is still lacking. The aim of this research was to evaluate the beta-cell reserve in non obese diabetic patients in relation to the metabolic control and the duration of disease. The main aim was to identify values of postprandial plasma C-peptide that can characterize the patients requiring insulin treatment. A daily profile was performed in 135 non obese diabetic patients, within 20% of their ideal body weight. The mean diurnal values (m) and the mean post-prandial increases (delta mpp) of plasma glucose (G), insulin (IRI) and C-peptide (CP) were assessed. Fourty-four patients showed good (NwD-GC: G-m = 138 +/- 3.2 mg/dl) and 91 poor metabolic control (NwD-SF: G-m = 210 +/- 4.8 mg/dl), according to the G-m lower or higher than 150 mg/dl. Beta-cell reserve (CP-delta mpp: 0.70 +/- 0.03 vs 1.39 +/- 0.04 ng/ml) and C-peptide/insulin molar ratio (CP/IRA-delta mpp: 2.36 +/- 0.06 vs 2.80 +/- 0.06) were significantly lower (p < 0.001) in NwD-SF than in NwD-GC. NwD-GC and NwD-SF were respectively divided into three subgroups, according to the duration of disease. A progressive reduction of CP-delta mpp and an increase in SF prevalence, from the first to the third decade of diabetes duration, was observed. In both NwD-Gc and NwD-SF the duration of disease inversely correlated with CP-delta mpp (NwD-GC: y = 1.59-0.019X, p < 0.001; NwD-SF: y = 1.01-0.023X, p < 0.001). The analysis of the two regression lines showed that patients with CP-delta values lower than 1.0 ng/ml require insulin treatment. In conclusion the duration of diabetes and the progressive reduction of beta-cell reserve represent the major pathogenetic factors in secondary failure.