The systolic time intervals (LVET, PEP, and ratio LVET PEP ) were determined in 53 patients presenting with signs or symptoms of thyroid dysfunction. Patients with clinical evidence for congestive heart failure, with arterial hypertension or old myocardial infarction, and patients receiving cardioactive drugs were excluded from the study. Thyroid function was evaluated by means of T 3-RIA, serum thyroxin, and TRH stimulation test. In patients with hyperthyroidism (untreated and on treatment), PEP c was shortened significantly (91.7 ± 11.0 msec. compared to a control of 118 ± 9.3 msex.). LVET c was prolonged only insignificantly. Hence, the ratio LVET c PEP c showed a significant increase in hyperthyroid patients. In hypothyroid patients (untreated and on treatment), PEP c was prolonged to 152.4 ± 15.1 msec. LVET c was shortened (399.7 ± 14.4 msec. compared to a control of 413.7 ± 15.9 msec.). The ratio LVET c PEP c was decreased. In seven patients, serial determinations of PEP c were performed; the values returned towards normal after successful treatment of thyroid dysfunction. A significant linear correlation between PEP c and T 4 (r = −0.67) as well as LVET c PEP c and T 4 (r = 0.64) was found. In 17 patients with hyperthyroidism and in eight patients with hypothyroidism 33 determinations of PEP c were performed. All but one of the values differed by at least 1 SD from our normal group, or, taking 2 SD, thyroid function could be evaluated correctly in approximately 75 per cent of patients.