A hypothesis was tested that providing the breeder hens with exogenous thyroxine (T(4)) would help their offspring to better survive the ascites-inducing condition during the growing period. In total, 132 broiler breeder hens were randomly assigned to one of 3 treatments: control (CON), hypothyroid [HYPO; 6-N-propyl-2-thiouracil (PTU)-treated], and hyperthyroid (HYPER; T(4)-treated). The hens were artificially inseminated, and the hatching eggs (n = 1,320) were incubated. No eggs in the HYPO group hatched. The 1-d-old male chicks (n = 288) from other groups were reared for 42 d under standard or low ambient temperature to induce ascites. Blood samples were drawn from the hens, embryos, and broilers for determination of T(4) and triiodothyronine (T(3)). The hematocrit was also determined in broilers. The PTU-treated hens had an increased BW along with lower plasma T(3) and T(4) concentrations. Plasma T(4) was higher in the HYPER hens compared with CON hens, but T(3) concentration was not different between these groups. The fertility rate was not affected by either hypo- or hyperthyroidism. The embryos in the HYPO group had lower plasma T(3) and T(4) concentrations at d 18 of embryonic development and internal pipping. Higher plasma T(4) was recorded in the HYPER birds at internal pipping, although plasma T(3) concentration was not affected at this stage. Maternal hyperthyroidism decreased the overall incidence of ascites in the cold-exposed chickens (10.0 vs. 33.4% for HYPER and CON groups, respectively). Although the effect of maternal PTU or T(4) treatment on plasma thyroid hormones and on the right ventricle-to-total ventricular weight ratio in the broilers was not significant, the cold-exposed healthy CON chicks showed higher hematocrit values, compared with the HYPER birds. It was concluded that maternal hyperthyroidism could decrease the incidence of cold-induced ascites in broiler chickens; however, probable causal mechanisms remain to be elucidated.