Morphology and function of the left ventricle were evaluated by echo and Doppler examination in 16 females affected by Hashimoto's thyroiditis, aged 13.3 (4.5) years (range 7.9-24.6). They were on L-thyroxine (L-T4) treatment for a period of 2.8 (2.8) years (range 0.8-7.6) with a mean daily dose of 77 (18) micrograms/m2. Left ventricular mass, systolic and diastolic function, cardiac output and systemic vascular resistance did not differ from a control group matched for age, sex and body size. A further analysis of the patients according to thyrotrophin serum levels (less or more than 0.1 mU/l) gave similar results. Moreover, no relationship was found between echocardiographic findings and age, L-T4 daily doses, duration of treatment and serum level of thyroid hormones. We can therefore conclude that chronic L-T4 treatment for Hashimoto's thyroiditis at the given doses did not affect cardiac function and morphology in children and adolescents; however, a longer follow-up is needed before confirming the safety of this therapy in the long term.