Introduction: It is well documented that high blood homocysteine (Hcy) levels possibly contribute to insulin resistance through the induction of resistin and inhibition of adiponectin expression in mouce adipose tissue in vitro. Additionally, in humans, hyperhomocysteinemia has been recognized as an independent risk factor for cardiovascular disease (CVD). Aim of the study: Our study aimed at the evaluation of the relationship between total plasma Hcy levels and physical activity, cardiorespiratory fitness and dietary habits in young healthy men. Material and methods: Plasma Hcy levels, macronutrient intake, maximal oxygen uptake and physical activity were assessed in 65 volunteers. Physical activity was assessed using a Seven-Day Physical Activity Recall (SDPAR), maximal oxygen uptake was measured on bicycle ergometer, and plasma Hcy was determined with the fluorescence polarization immunoassay (FPIA). Dietary habits were evaluated from dietary records collected over 4 days. On the basis of median Hcy levels (9.7 ?mol/l) two groups of subjects were separated - with plasma Hcy levels lower and higher than median value (Group LH and HH, respectively). Results: Group LH and HH did not differ with respect to maximal oxygen uptake or physical activity level. The only difference between groups with different circulating Hcy was the percentage of energy derived from protein - slightly but significantly higher in LH than in HH subjects (16% and 13% of total energy, respectively). Conclusions: At least in our participants neither cardiorespiratory fitness nor physical activity affected circulating Hcy. However, higher percentage of energy derived from protein had a beneficial effect on plasma Hcy.