The aim of the present study was to investigate plasma levels of asymmetric dimethylarginine (ADMA), an important endogenous inhibitor of nitric oxide synthase, in populations at high risk for atherosclerosis as compared to healthy controls, and furthermore to evaluate the effect of cholesterol lowering therapy in individuals with hypercholesterolemia. The present study consisted of 32 men with untreated hypercholesterolemia (HC group), 38 individuals with well-controlled insulin-dependent diabetes mellitus (DM group) and 20 healthy individuals (controls). The HC subjects were randomly allocated into a double blinded, placebo-controlled cross-over designed study with 8 weeks treatment with pravastatin (40 mg/day) or matching placebo. ADMA levels were statistically significantly higher in DM and HC individuals as compared to controls (P<0.001 for both), and the L-arginine/ADMA ratios were significantly lower in both groups (P<0.001 and P<0.005, respectively). Significant reductions in total cholesterol (TC) and LDL-C levels on pravastatin were obtained (P<0.001 for both), whereas no changes were observed in the levels of ADMA or the L-arginine/ADMA ratios. Statistically significant correlations between ADMA and TC and LDL-C were found (r=0.41, P<0.001 for both). In conclusion, significantly elevated ADMA levels and reduced L-arginine/ADMA ratios were found in individuals with diabetes type-1 as well as in hypercholesterolemia. Treatment with pravastatin 40 mg/day for 8 weeks had no effect on the levels of ADMA in hypercholesterolemic men.