Two female monozygous twin pairs discordant for anorexia nervosa are reported, and literature reports on 18 other monozygous pairs are reviewed. Four of the reported pairs are inadequately described and there is doubt about the diagnosis. When the 2 pairs reported here are included, 16 pairs remain: l (38%) were concordant and 10 discordant. Only 3 pairs fulfilled rigorous criteria for anorexia nervosa and zygosity and one of the pairs was concordant. The population prevalence is probably about 2 promille for hospitalized cases, and there is a 6% risk for female sibs of anorectic probands. Hence, if about 1/3 of monozygotic pairs are in fact concordant, a role of genetic factors in the etiology may be suggested.