Familial medullary carcinoma of the thyroid gland is often associated with phaeochromocytoma; this condition is known as Sipple syndrome. The rate of progression of the disease and the degree of malignancy of the medullary carcinoma may vary from family to family. Thus the nature and behaviour of this disease in any particular family influences the management of the thyroid carcinoma component. The authors measured plasma calcitonin and plasma catecholamines in 21 members of a family with Sipple syndrome. In one branch of this family 5 out of 7 members have so far been affected, 4 with metastases or recurrence of the carcinoma and 2 with development of phaeochromocytomas. In contrast, of the 14 members in the other 4 family branches, only 4 have had pathologically elevated plasma calcitonin concentrations following alcohol provocation, probably representing C-cell hyperplasia or very early neoplasia and apparently not progressing. None has evidence of phaeochromocytoma. This family demonstrates a striking variability in the natural history of Sipple syndrome which, despite increasing discovery of more families with medullary thyroid carcinoma, has not been previously reported. In the light of this finding, a reappraisal of the management of the thyroidal component of this disease is necessary. It appears that radical neck surgery may be needed to prevent recurrence of the disease.