Between 1986 and 1989 172 patients were operated on thyroid cancer. Twenty-nine (17%) of them had a medullary carcinoma. In 20 of these patients (69%) the carcinoma occurred in a sporadic and in 9 (31%) patients in a familial form. Eighteen patients (62%) had to be operated because of tumor recurrence and in 4 of them additional surgery was necessary because of distant metastases. Due to tumor recurrence 7 patients underwent multiple operations during the above period. Adequate first operation was performed only in 6 patients (33%). Based on family screening a carcinoma was diagnosed in 4 patients in its occult stage. Diagnostic methods include biochemical and imaging methods. Calcitonin is the most sensitive tumor marker. Basal and stimulated serum calcitonin analysis provides a very efficient method to detect medullary carcinoma in early tumor stage and to treat the disease curatively. DNA-analysis improves early diagnosis of persons at risk.