Homocysteine is a risk factor for vascular diseases, and lowering of plasma total homocysteine (tHcy) may be beneficial for health. Homocysteine can be remethylated to methionine by betaine–homocysteine methyltransferase using betaine (2(N,N,N-trimethyl)glycine) as methyl donor. A dose of 6 g betaine/d has been used in the treatment of homocystinuria, but data on the dose–response are scarce. Thirty-four healthy men and women were supplied with doses of 1, 3 and 6 g betaine and then with 6 g betaine + 1 mg folic acid for four consecutive 1-week periods. The mean plasma tHcy concentration decreased by 1·1 (NS), 10·0 and 14·0 % (P<0·001) after supplementation with 1, 3 and 6 g betaine respectively. A further decrease in plasma tHcy by 5 % (P<0·01) was achieved by combining 1 mg folic acid with the 6 g betaine dose. Plasma betaine increased from 31 (SD 13) to 255 (SD 136) μmol/l in a dose-dependent manner (R2 0·97). We conclude that plasma tHcy is lowered rapidly and significantly by 3 or 6 g betaine/d in healthy men and women.