We here present a family where three individuals in three generations had varying degrees of goiter, tachycardia, fatigue, hyperactivity, and learning disability. Serum T3 and free T4 were elevated, whereas TSH was normal or slightly increased. The clinical findings in combination with the hormone values led to several supplementary investigations and therapies being carried out, but they had no beneficial influence on the patients' symptoms. The commonest form of thyroid hormone resistance (RTH) is an autosomal dominantly inherited disorder with varying degrees of hypo- and hyperthyroidism, including the hormonal changes described above. Several mutations, particularly in exons 9 and 10 of the thyroid hormone receptor beta gene, have been described and shown to be responsible for RTH. Exons 7, 8, 9, and 10 in the thyroid hormone receptor beta gene were amplified by polymerase chain reaction and analyzed by DNA sequencing. A heterozygous point mutation in nucleotide 1244 in exon 9 was demonstrated in the two patients with RTH that were available for the study. The guanidine to thymidine point mutation changed the codon for arginine in position 320 in the receptor protein to leucine. This mutation has previously been shown to decrease receptor affinity for T3; it has been demonstrated in some patients with RTH, and it is probably the cause of RTH in the family described in this study.