BACKGROUND As diabetes mellitus represents a situation in which production of peroxides is increased, the aim of this study was to investigate the relationship between plasma and platelet levels of ascorbic acid (AA)/dehydroascorbic acid (DHA) and those of malonyldialdehyde (MDA), an indirect marker of lipoperoxides, both assayed using high-performance liquid chromatography (HPLC), in 59 patients with insulin-dependent diabetes mellitus (IDDM) compared with 51 healthy control subjects matched for sex, age, smoking habits, as well as for dietary intake of energy, alcohol and vitamin C. RESULTS Mean plasma and platelet MDA were significantly higher in the patients affected with IDDM than in control subjects. Moreover, the diabetic group was characterized by a huge decrease in plasma AA [8.45 +/- 5.5 mumol L-1 (SD) vs. 33.4 +/- 7.6 mumol L-1, P = 0.0001], mirrored by a significant increase in plasma DHA (11.9 +/- 3.9 mumol L-1 vs. 3.9 +/- 2.5 mumol L-1, P = 0.0001). No detectable DHA was observed in the platelets from both diabetic and control subjects, whereas AA was significantly increased in platelets from diabetic patients compared with control subjects (42.6 +/- 7.4 vs. 34.8 +/- 5.1 nmol 10(-9) platelets, P = 0.0001). Platelet AA in the diabetic group was significantly inversely correlated with glycated haemoglobin (r = -0.34; P = 0.04) and directly with plasma AA (r = 0.39; P = 0.02), the sum of plasma AA + DHA (r = 0.44; P = 0.009) and with platelet MDA (r = 0.38; P = 0.02). CONCLUSIONS (a) The ratio plasma AA/DHA is significantly lowered in IDDM in association with an increase in MDA levels; (b) only AA is detected in platelets, being augmented in the diabetic group; (c) plasma ascorbate depletion does not reflect platelet levels of AA; and, finally, (d) metabolic control, as well as intracellular lipoperoxides, modulates platelet AA in IDDM.