High Homocyst(e)ine levels (H) have been recently recognized as a risk factor for atherosclerosis. Patients with Diabetes Mellitus (DM) are prone to atherosclerosis. Therefore, this study was designed to search for the effect of DM on H and their relationship. Forty-one Type 1 diabetic subjects (DS, age 34.8 +/- 12 yr, DM duration : 10.7 +/- 11.1 yr) were compared to 40 age-matched control subjects (CS, age 34.2 +/- 9.1 yr). H (measured by ion-exchange chromatography, units : mu mol/l) and several parameters (creatininaemia; triglycerides; total, HDL, LDL cholesterol; Lp(a); HbA1c; vitamins B9 and B12) were determined after an overnight fast. H were significantly (p = 0.0001) lower in DS (6.8 +/- 2.2) than in CS (9.5 +/- 2.9). This difference was still apparent in male and female subgroups compared to matched CS (p = 0.003 for each). No correlation was found between H and : lipids, vitamins, renal or retinal status. But H seemed to increase with age, especially in women (p = 0.03; r = 0.32). While there is, at this time, no explanation for the lower H observed in DS, it appears that H cannot directly account for accelerated atherosclerosis in DM. Nevertheless, it remains to be established if high, or even normal, H could identify a subgroup of DS at higher risk of precocious and severe atherosclerosis.