To study serum thyroglobulin (Tg) levels in patients with thyroid disorders compared to sex- and age-matched control subjects and to correlate the Tg levels to the thyroid function, 71 patients were investigated before treatment was started. Serum Tg, measured by a double antibody radioimmunoassay, was elevated in all groups with thyroid disorders, as compared to their controls, but the values showed large overlaps between groups. The highest median values were seen in the two groups of patients with toxic goitres (toxic adenoma and Graves' disease). The Tg values in patients with non-toxic goitres (diffuse and nodular) and in controls showed a log normal distribution, whereas the distribution of values from patients with toxic goitres was different. No correlation was found between serum Tg and serum thyroxine, serum triiodothyronine and serum TSH, respectively. It is concluded that determination of serum Tg is of little diagnostic value in thyroid diseases.