Objective We evaluated the effect of a long-term, high-copper intake on plasma total homocysteine, folate, vitamin B12, and pyridoxal-5′-phosphate concentrations in humans. Methods Nine healthy young men were confined in a metabolic research unit for 18 d and fed 3-d rotation diets supplying an average of 1.6 mg of copper per day followed by 129 d of free-living conditions when they received 7 mg of copper per day in addition to their usual diets. The subjects returned to the metabolic research unit for the second 18-d period and were given the same diets as during the first 18 d with the exception that the copper intake was 7.8 mg/d. There was no apparent biochemical indication that the subjects were deficient in copper before the large-dose copper intake. Blood samples were obtained at the end of the first and second 18-d periods at the metabolic research unit, and plasma concentrations of total homocysteine, folate, vitamin B12, and pyridoxal-5′-phosphate were measured. Results The long-term, high-copper intake resulted in small but significant decreases in plasma concentrations of total homocysteine and folate. There was no effect of the high-copper intake on plasma concentrations of vitamin B12 and pyridoxal-5′-phosphate. Conclusions These findings can be explained by our previous observation in rats suggesting that methionine synthase is copper dependent and that the metabolism of homocysteine and folate is regulated in part by copper nutriture. It may be necessary to consider copper nutriture for the interpretation of plasma concentrations of total homocysteine in humans. © Elsevier Inc. 2004.