A family with MEN IIa (medullary thyroid carcinoma (MCT), pheochromocytoma and hyperparathyroidism) was identified. Three relatives had been treated for MCT earlier. Eleven asymptomatic family members had elevated pentagastrin (PG)-stimulated serum immunoreactive calcitonin (i-CT) concentrations, including one who earlier had a pheochromocytoma removed. Nine of these subjects underwent thyroidectomy, and histological examination revealed multifocal MCT in all. Although surgery was judged complete in all, elevated PG-stimulated serum i-CT levels postoperatively indicated residual disease in 5. The natural history of MCTs in the present family varied, with most cases behaving benignly. Occasionally, however, the disease pursued an aggressive course. As MCT often metastasizes before being clinically evident, high cure rates can only be obtained by early diagnosis and treatment, possibly in the pre-metastasizing phase of C-cell hyperplasia, detectable only by elevated PG-stimulated serum i-CT levels.