A 62 year-old man presented with massive pericardial effusion and a thyroid nodule. One thousand ml of serosanguinous pericardial effusion was tapped and the cytology consistent with metastatic papillary carcinoma. Total thyroidectomy was done. The pathological report of the thyroid gland was follicular carcinoma, poorly differentiated with capsular and vascular invasion. The patient received a treatment of 150 mci I-131 without complications. The total body scan after I-131 treatment showed only residual thyroid tissue. There was no demonstrable abnormal uptake in other parts of the body. No significant reaccumulation of pericardial fluid occurred. The diagnosis was follicular carcinoma of the thyroid with pericardial metastasis. The patient was discharged from the hospital with an improved clinical status.