The aim of the present study was to evaluate the relationship of C-peptide and the C-peptide/bloodsugar ratio with clinical/biochemical variables presenting a well-known association with insulin resistance in NIDDM patients in acceptable control, obtained without the use of exogenous insulin. A total of 118 non insulin dependent diabetes mellitus (NIDDM) patients treated with diet/oral drugs and having a HbA(1c) level < 7.5% have been studied. Non-stimulated C-peptide levels (RIA) and the C-peptide/bloodsugar ratio have been determined and their relationships with the blood pressure status, blood pressure figures, estimates of adiposity, age, known duration of diabetes, current therapies, plasma lipids, glycaemic control, urinary albumin excretion rate, uric acid and creatinine have been ascertained. C-peptide levels were significantly (P < 0.05) correlated with systolic (r = 0.21) and diastolic blood pressure (r = 0.19), BMI (r = 0.21), high density lipoprotein (HDL) (r = -0.22), non-HDL-cholesterol (r = 0.23), apolipoprotein B (r = 0.29), log of triglycerides (r = 0.39) and uric acid (r = 0.35). The C-peptide/bloodsugar ratio had statistically significant correlations with known duration of diabetes (r = -0.23), diastolic blood pressure (r = 0.21), body mass index (BMI) (r = 0.22), log of triglycerides (r = 0.23) and uric acid (r = 0.36). Hypertensives had higher C-peptide levels than normotensives (1.04 +/- 0.04 versus 0.88 +/- 0.04 nmol/ml, respectively (mean +/- S.E.), P < 0.05) and this statistically significant difference remained after adjustment for age and known duration of diabetes. In well-controlled NIDDM patients not receiving exogenous insulin, both C-peptide levels and the C-peptide/bloodsugar ratio have statistically significant relationships with clinical/biochemical variables presenting a well-known association with insulin resistance.