Background: Thyroid dysgenesis (TD) is usually sporadic.In approximately 2 % -8 % of TD cases, familial TD has been identifi ed. Aims: The aim of this study is to defi ne the prevalence of thyroid developmental anomalies in fi rst-degree relatives of children with TD-caused congenital hypothyroidism (CH). Methods: The investigation included 102 relatives of 33 children with CH and TD (study group) and 27 relatives of 12 normal children (comparative group). All the individuals were subjected to thyroid ultrasound and serum thyroid stimulating hormone (TSH) and free T4 (FT4) determinations. Statistical analysis was based on Fisher ' s exact test. Results: TD-caused familial CH was noted in 2 of 33 (6 % ) children with CH. Asymptomatic thyroid developmental anomaly was seen in 1 of 102 (1 % ) relatives -left thyroid lobe hypoplasia in the mother of a girl with CH and thyroid severe hypoplasia. Familial prevalence of asymptomatic TD in the study group was observed in 1 of 32 families (3.13 % ). None of the comparative group members demonstrated any thyroid developmental anomalies. Conclusions: The prevalence rate of thyroid developmental anomalies in the study group is slightly higher than in the comparative group. These disturbances are asymptomatic.