Feeding is subjected to circadian regulation; therefore, changes in the components of the endogenous oscillator regulating circadian rhythms may be involved in disordered rhythmicity of eating behavior as it occurs in anorexia nervosa (AN) and bulimia nervosa (BN). We investigated whether the 3111T/C polymorphism of the CLOCK gene, which is part of the endogenous oscillator system, was associated to AN and/or BN. A total of 241 women, including 90 healthy controls, 60 patients with AN and 91 patients with BN, participated into the study. The frequencies of 3111T/C genotypes and alleles did not significantly differ among the groups. In both the AN and BN group, subjects carrying one copy of the C allele had a lifetime body weight significantly lower than those carrying the T/T genotype. These findings, although preliminary, suggest that the 3111T/C polymorphism of the CLOCK gene does not play a major role in the genetic vulnerability to AN and BN, but it seems to predispose eating disorders (EDs) patients to a more severe lifetime body weight loss. © 2007 Wiley-Liss, Inc.