A retrospective search for lymphocytic infiltration of the thyroid was performed on archival paraffin specimens from 95 cases of thyroid medullary carcinoma observed at a single institution during a 30-year period. A mild lymphocytic infiltration of the nonneoplastic thyroid tissue, mainly concentrated at the edge of the tumor mass, was observed in 33 cases, while in a further 24 cases a moderate to marked lymphocytic infiltration, resembling that seen in chronic lymphocytic thyroiditis, was homogeneously distributed all over the gland, with no apparent relationship to the site of the primary tumor. Virtually no lymphocytic infiltration was detected inside of any of the tumors. The presence of chronic lymphocytic thyroiditis seemed to be a characteristic of the host rather than a tumor-induced event. Statistical univariate analysis of relapse-free survival and overall survival showed that lack of chronic lymphocytic thyroiditis marked those patients with an increased risk of disease recurrence and death. However, the observed risk for survival was statistically nonsignificant, whereas that for disease recurrence was significant and remained in a multivariate model of statistical analysis.