Serum thyroid hormones, serum thyroxine-binding proteins and serum thyroid hormone fractions have been measured in children with congenital heart disease before and after open cardiac surgery. Twenty prepubertal patients, mean (+/- SD) age 3.6 +/- 3.7 yr, were studied before, immediately after, and 24 and 48 h after surgery. A control group of 6 normal prepubertal children was also studied in basal conditions. Serum TSH was normal in all samples collected. Significantly low mean levels of serum TBG (261 +/- 57 vs 456 +/- 71 nmol/L in normals), serum TBPA (2692 +/- 1119 vs 5999 +/- 2226 nmol/L), serum TBG-bound T4, serum TBPA-bound T4, serum TT3, serum TBG-bound T3 and free T3 were found before cardiac surgery in the patients. While serum binding proteins did not change after surgery, significant decrements in serum TT4, serum TBG-bound T4, serum TT3, serum TBG-bound T3, serum albumin-bound T3 and free T3 were observed after surgery. Free T4 and albumin-bound T4 remained normal. Our study shows that many features of nonthyroidal illness were present in our patients before surgery. In this context, the stress of surgery induced further alterations in several parameters of thyroid metabolism. It is concluded that the changes occurring in this model of chronic, as well as acute, nonthyroidal illness reflect adaptative changes, rather than altered thyroid function, as shown by normal serum free T4, serum albumin-bound T4 and serum TSH.