The estrogen receptor (ER) plays an important role in mediating estrogen action on target tissues. ER-alpha, the most abundant, is found in all human reproductive tissues and studies on alpha-ER knockout mice have highlighted its role in reproduction. ER-alpha gene (ESR1) polymorphisms have been associated with a variety of disorders including human infertility. In this study, we examined the association of ESR1 PvuII and XbaI polymorphisms with fertility in two populations with different reproductive patterns and precisely in a sample of healthy Italian men and women (n = 178) and in a sample of healthy African-Ecuadorian women (n = 57). ESR1 xx and ppxx genotypes among the Italian men were found to be associated with an above-median number of children (P = 0.01 and P = 0.004, respectively). ESR1 pp genotype among the Italian women showed a tendency to be associated with a lower number of abortions (P = 0.04), whereas ESR1 pp and ppxx genotypes among African-Ecuadorian women were associated with a higher number of children (P = 0.02 and P = 0.03, respectively). These results are consistent with previous observations indicating a role of ESR1 genotypes in human infertility and give insight into the complex interactions between genotypes and reproductive behaviours in human populations.