Thirty-nine patients were studied for 13 years following resection of non-toxic goitre in order to correlate changes in serum thyroglobulin (Tg) to relapse. Preoperative serum Tg was elevated. After operation, mean serum Tg declined to 43 micrograms/L at one year and increased to 90 micrograms/L at 10 years. Thyroid hormones showed evidence of compensated hypothyroidism at one month which normalised within one year. 13 years after resection thyroid volume was determined by ultrasonography in 30 patients. In 10 patients the thyroid volume was enlarged (> or = 28 ml). In this group delta Tg after resection was 133 micrograms/L, compared to 26 micrograms/L in the 20 patients without relapse (volume < 28 ml). Thus, repeated serum Tg determinations may provide biochemical evidence of increased growth of thyroid remnants following goitre resection.