Second primary neoplasms are occurring with increasing frequency. Despite growing literature on the incidence and etiology of this phenomenon, very little has been documented about the clinical aspects and biological behavior of these tumors. As our experience has hinted at a worse prognosis for second primaries, we thought it of interest to study this phenomenon for one type of tumor. We reviewed 32 cases of thyroid cancer arising as a second neoplasm in patients with Hodgkin's disease and compared various clinical characteristics to reported series of de novo (non-radiation-induced) thyroid cancer. Thyroid cancer, as a second primary, occurred more frequently in males with a more malignant histology and was diagnosed at a more advanced stage. The survival appears to be worse. The behavior of radiation-induced thyroid cancer in a host with prior malignant neoplasia appears to be more aggressive than that of both de novo (non-radiation-induced) thyroid cancer and radiation-induced thyroid cancer in a host with no prior malignancy. The selected nature of the cases precludes any firm conclusions. This type of information should be collected prospectively for all secondary malignancies as it may have an impact on the clinical management of these patients.