Background: A high plasma total homocysteine concentration is associated with increased risk of cardiovascular disease. Consumption of unfiltered or filtered coffee raises total homocysteine concentrations in healthy volunteers. The responsible compound, however, is unknown. Objective: The objective was to determine whether caffeine explains the homocysteine-raising effect of coffee. Design: Forty-eight subjects aged 19-65 y completed this randomized crossover study with 3 treatments, each lasting 2 wk. Subjects consumed 6 capsules providing 870 mg caffeine/d (test treatment), 0.9 L paper-filtered coffee providing ≈870 mg caffeine/d, or 6 placebo capsules. Blood samples were drawn fasting and 4 h after consumption of 0.45 L coffee or 3 capsules. Results: The mean fasting plasma homocysteine concentration after the placebo treatment was 9.6 ± 3.1 μmol/L. The caffeine and coffee treatments increased fasting homocysteine by 0.4 μmol/L (95% CI: 0.1, 0.7; P = 0.04), or 5%, and by 0.9 μmol/L (95% CI: 0.6, 1.2; P = 0.0001), or 11%, respectively, compared with placebo. The increase in homocysteine concentrations 4 h after consumption of 0.45 L coffee relative to consumption of 3 placebo capsules was 19% (P = 0.0001). Caffeine treatment had a much weaker acute effect on homocysteine (4%; P = 0.09). Effects of caffeine were stronger in women than in men, but the effects of coffee did not differ significantly between men and women. Conclusions: Caffeine is partly responsible for the homocysteineraising effect of coffee. Coffee, but not caffeine, affects homocysteine metabolism within hours after intake, although the effect is still substantial after an overnight fast. Chemicals/CAS: Caffeine, 58-08-2; Homocysteine, 454-28-4; Placebos; Vitamin B 12, 68-19-9; Vitamin B 6, 8059-24-3