The diagnosis of papillary carcinoma of the thyroid metastatic to the lung frequently requires a battery of noninvasive tests. Occasionally, invasive procedures such as open lung biopsy, transthoracic needle biopsy, and transbronchial lung biopsy are employed to confirm the diagnosis. A 31-yr-old woman with papillary thyroid carcinoma treated previously by a near-total thyroidectomy and 131I ablation presented to our clinic with shortness of breath and a clear chest roentgenogram. A post-131I treatment whole body scan revealed widespread 131I pulmonary uptake, and the presence of papillary thyroid cancer was confirmed by bronchoalveolar lavage. We conclude that bronchoalveolar lavage should be considered when tissue confirmation of metastatic papillary carcinoma to the lung is needed. During the evaluation and follow-up of this patient, we were able to determine that metastatic papillary carcinoma to the lung may cause a methacholine bronchoprovocation test to be falsely positive for asthma.