Presymptomatic screening of medullary thyroid carcinoma in MEN IIA families enables the early diagnosis of this tumor with its significant morbidity. Biochemical screening consists of basal and stimulated serum calcitonin evaluation. Genetic screening is based on DNA analysis using linked DNA markers. Thyroidectomy at an occult tumor stage may be curative. Calcitonin measurement was carried out in 58 apparently unaffected family members at risk and 11 MEN IIA patients. Calcitonin elevation was detected in nine individuals. All nine underwent thyroidectomy. Histologic examination confirmed medullary thyroid carcinoma in eight patients and in one case C cell hyperplasia. Postoperatively, eight patients (89%) are clinically and biochemically tumor-free (mean follow-up 30 months). DNA screening results in one affected family are presented. DNA analysis allowed recognition of one apparently unaffected individual at risk as a MEN IIA gene carrier. One family member at risk was found not to carry the gene and may be excluded from further screening.