AIMS: There is evidence of an excess of acute cardiovascular (CV) events in marathon runners. High plasma total homocysteine (tHcy) concentrations are a recognised risk factor for CV events. Therefore, we investigated the changes in plasma tHcy concentrations 24h before and after a marathon race. METHODS AND RESULTS: Twenty-two non-professional male athletes, mean age 35.6 (6.6), range 23-49 years, were studied the day before and 24 h after finishing a marathon race. None of the athletes was a carrier of the MTHFR 677TT genotype and no ingestion of supplements of vitamins (B12, B6, folic acid) was allowed. RESULTS: Changes in plasma folate and plasma vitamin B12 concentrations were not detected post-race, but a significant increase in plasma tHcy concentrations was demonstrated. Plasma tHcy increased 19% 24h after the race. Before the race 20% of the subjects had a plasma tHcy concentration > 10 micromol/l (cut-off point for ischaemic heart disease risk), while after the race 50% had plasma tHcy concentrations> 10 micromol/l. CONCLUSION: An increase in plasma tHcy concentrations was observed after a marathon race in non-professional not well-trained male athletes performing strong physical activity. The potential physiological or pathological implications of this finding are unknown.