We investigated a possible influence on birth weight in Turner's syndrome of many clinical, hormonal, genetic and immunogenetic variables. We considered 97 patients with Turner's syndrome. Patients with parents with identical GM (Gamma heavy chains Marker) phenotype had a significantly lower birth weight than those with parents with different GM phenotype. Karyotype other than 45,X, HLA (Human Leukocyte Antigen) parental sharing, mother-patient compatibility and elevated 17-hydroxyprogesterone (17OHP) serum level after adrenocorticotropin hormone (ACTH) and absence of heart and kidney malformations and lymphedema were associated with a lower birth weight, but not significantly. Multiple interactions showed that the presence of an identical GM phenotype in parents, together with other conditions (karyotype other than 45,X, adrenal dysfunction, HLA parental sharing, mother-child compatibility, KM(3) [Kappa light chains Marker] phenotype) resulted in a further decrease of birth weight. These data might suggest a negative effect of genetic similarity on intrauterine growth in Turner's syndrome.