Hormonal and clinical data on 38 tests performed in 7 hypothyroid patients given substitutive treatment with L-thyroxin were analysed. The aim of the survey was to assess the dependability of the various parameters used to estimate dose adequacy in substitutive treatment. Examination of individual cases revealed that overdoses of thyroxin increased T4 to above normal limits without increases in T3, FT3, FT4 or hypophyseal suppression. Chronological discrepancies between clinical data and normal parameters were also noted. However on the data as a whole significant correlations were noted between T4 and TSH (p less than 0.001), between T4 and delta-TSH (p less than 0.001), between T3 and TSH p less than 0.01), between FT3 and delta-TSH (p less than 0.01), between FT4 and delta-TSH (p less than 0.01). FT4 and delta-TSH were also significantly correlated (p less than 0.02) with the Billewicz clinical index of hypothyroidism. It is concluded that the clinical and hormonal data in treated hypothyroid cases have the same significance as in normal or untreated hypothyroid cases. However, it must be borne in mind that unusual relations between the various thyroid hormone fractions including TSH and the standardised clinical examination may arise while doses are being established. In this case the data cannot be automatically interpreted but merely carefully assessed in the light of experience.