Out of 450 patients referred for thyroid scintigraphy 83 consecutive patients with nodular goitre were selected for dual scintigraphy with 99mTc and 123I. Seventy-four patients had one or more palpable nodules and the other nine patients had palpably diffuse goitres with one or more nodules on the routine 99mTc scintigram. For the comparison of scintigrams an eight level colour display was used, and regional differences between the 99mTc and 123I scintigrams were defined as a minimum of 25% (2 colour levels) variation of the relative activity in well defined areas. Thirteen patients, i.e. 16% of those with nodular goitre, had discordant scintigrams. All 13 patients had non-malignant thyroid disorders, (simple goitre, radio-iodine treated hyperthyroidism, autonomous adenoma, subacute thyroiditis, Riedel's and lymphocytic thyroiditis). The diagnosis was verified histologically in eight of these patients. Eight of the 13 patients had a high 99mTc activity relative to 123I, five patients showed the opposite discrepancy. Another 5 patients with a follicular or mixed follicular thyroid carcinoma had identical scintigrams. These findings indicate that 123I does not offer such diagnostic advantages over the cheap and readily available 99mTc, that its use for routine scintigraphy of the thyroid gland seems to be justified at present.