This study was conducted to evaluate the analytical performance of a new colorimetric assay for plasma homocysteine (Hcy) and to determine the influence of the method used on the frequency of hyperhomocysteinemia and on the association between Hcy and biological parameters in diabetic patients. Plasma Hcy was measured in 160 diabetic patients by high performance liquid chromatography (HPLC) in parallel with an automated colorimetric assay (DiaSys Diagnostic Systems GmbH) using the Olympus AU640 and Abbott C8000 analyzers. Plasma glucose, creatinine, folates, vitamin B(12), high-sensitivity C-reactive protein (hs-CRP), and blood hemoglobin A1c (HbA1c) concentrations were measured by routine methods. The Hcy concentrations obtained with the HPLC (x) and colorimetric methods (y) were highly correlated (y = 1.032x - 0.65, r(2) = 0.884 for AU640; y = 1.028x + 0.87, r(2) = 0.934 for C8000). The frequency of Hcy >15 micromol/L was 41.9% by HPLC assay and there was no statistically significant difference using the colorimetric method. Patients with Hcy >15 micromol/L compared to < or =15 micromol/L using HPLC had significantly lower glycemia (p <0.001), lower HbA1c (p <0.001), lower estimated glomerular filtration rate (GFR) (p <0.0001), and lower vitamin B(12) levels (p <0.001). All these relationships were also noted when using the colorimetric assay on the Olympus AU640 or Abbott C8000 analyzers. In summary, this study shows that the DiaSys colorimetric assay is suitable for plasma Hcy determinations in routine clinical biochemistry.