Eleven patients with head and neck cancer (ten men, one woman; mean age, 65 years) (larynx, six; oropharynx, two; tongue, one; skin, one; thyroid, one) with regional lymph node enlargements either in contiguity or firmly adherent to the vascular structures of the neck have been treated by means of external microwaves (915, 434 MHz) applicators. All patients were treated by hyperthermia (42-45 degrees C) alone (ten sessions, twice a week, each lasting 30 minutes). Thyroid hormones (T4, T3, FT4, FT3, rT3, thyroid stimulating hormone less than obTSH]) were evaluated during sessions 1, 2, 3, and 8, just before the session (time [t] = 0) and at 10-minute intervals during the heating (t = 10, t = 20, t = 30). Blood was also taken 10 (t = 40) and 30 minutes (t = 60) after the end of each session. T4 showed a decreasing trend; T3 decreased significantly from t = 10 and reached the lowest values at t = 40 and t = 60; FT4 decreased at t = 40 and t = 60 at all sessions; FT3 and rT3 showed no change; TSH decreased at t = 10 until t = 40.