The proven power of DNA ploidy to predict mortality risk in medullary thyroid carcinoma (MTC) may be weakened when analyzed in conjunction with calcitonin immunoreactivity (CI) and amyloid staining (AS) of tumors. In this study 12 prognostic variables, including DNA ploidy, CI, and AS, were studied in 65 patients with MTC (57 sporadic; mean age 51 years) treated during 1946 through 1970. Cause-specific mortality rates at 10 and 15 years were 15% and 26%, respectively. By univariate analysis, TNM stages III or IV (p less than 0.0001), tumor unresectability (p less than 0.0001), male sex (p = 0.019), negative AS (p = 0.032), and low CI (p = 0.033) were significant predictors of increased mortality rates. DNA ploidy (p = 0.058) and inheritance pattern (p = 0.25) were nonsignificant. By multivariate analysis, only TNM stage, tumor resectability, and AS were independently significant (p less than 0.005). A prognostic model was created, based on presence or absence of these independent risk factors, and four risk groups were defined, capable of predictably defining mortality rates in MTC (p less than 0.0001). The model requires validation in larger series and independent verification by others. However, we believe that a risk-group scheme for MTC based on AS, disease stage, and completeness of tumor resection may have wide applicability and prove relevant to clinicians treating this disease.