We confirmed the relation between maternal weight and serum alpha-fetoprotein concentration and have shown a further relation--to ethnic origin. Of these two, maternal weight is the more closely related. Oriental, white, and Hispanic women showed no significant differences in serum AFP concentrations when corrections for maternal weight were applied. Black women showed consistently higher values, by an average of 10% at each week of gestation. These corrections only affected values falling just above or below the 95th centile cutoff. We conclude that corrections for maternal weight and race should be applied when values for alpha-fetoprotein in maternal serum are being interpreted.