Samples from a biological serum bank taken up to 23 years prior to diagnosis of thyroid carcinoma were analysed for human thyroglobulin, thyroid-stimulating hormone and thyroxin. After exclusions, the final study material consisted of 59 cases of papillary and follicular carcinomas. These cases were compared with 164 controls, matched for sex, age and time of sample taking. The most interesting finding was that concentrations of thyroglobulin in serum were abnormally elevated in cases compared with controls, equal to or above 30 microg/L, with odds ratio 7.0 (CI 3.1-15.7). This elevation of serum thyroglobulin occurred in 44% of the carcinoma cases. Sensitivity was around 50 for measurements taken up to 15 years prior to diagnosis, but 21 when the interval was over 15 years. Specificity was 89. No differences were found between cases and controls in values for thyroid-stimulating hormone and thyroxin.