Sixty-one patients with medullary thyroid carcinoma were investigated to establish relationships between the neoplasm's DNA content, other clinicopathologic factors, and patient survival (at least 5 years' follow-up). There were 26 (42.6%) familial and 35 (57.4%) sporadic carcinomas. Forty-two neoplasms (68.8%) were diploid and 19 (31.2%) were aneuploid by flow cytometric assessment. In univariate analysis, age, hereditary background, DNA content, calcitonin immunoreactivity, and type of surgery were strong predictors of clinical outcomes. In multivariate regression analysis, none of these factors was an independent prognosticator. Patients' gender, clinical stage, histologic subtype, and amyloid content of the tumor did not influence survival.