Findings on thyroid scintigram were compared with results of thyroid palpation and size and location of C-cell disease in 68 thyroid lobes of 35 patients with familial medullary thyroid carcinoma (MTC) and high basal or stimulated plasma calcitonin values. Rectilinear scans showed cold nodules in 24 (35%) of 68 lobes. In three lobes, the cold nodules did not coincide with MTC (2 macrofollicular adenomas, 1 colloid goiter). Thus, the true positive rate for rectilinear scans was 31%. Palpation identified nodules in 20 lobes (29%), but the true positive rate was only 26%. Patients with MTC have high basal or stimulated calcitonin values long before the tumor is detectable by scan or even later by palpation. When the tumors were large enough to be seen on thyroid scans, the most frequently encountered single pattern was that of symmetrically located cold nodules in the middle of otherwise normal thyroid lobes. This pattern, if encountered in a thyroid scan, should raise the suspicion of MTC.