The authors performed a prospective study to evaluate thyroid dysfunction in 130 patients with cancer who were receiving interleukin-2 (IL-2)-based immunotherapy. Primary hypothyroidism was the most common abnormality, occurring in 12% of patients before, 38% during, and 23% after immunotherapy. Hyperthyroidism occurred in 1%, 4%, and 7% of patients at those time intervals. Among patients initially euthyroid (n = 111), primary hypothyroidism developed in 32% during and 14% after immunotherapy, persisting a median of 54 days. Three patients required levothyroxine. Hyperthyroidism developed in 2% of patients during immunotherapy and 6% after. Thyroid dysfunction was not a function of sex, diagnosis, type of treatment, or response to immunotherapy. Elevated titers of antithyroglobulin and antithyroid microsomal antibodies were detected after treatment in 9% and 7%, respectively, of all patients without prior antibody abnormalities and did not correlate with response to therapy. The high incidence of therapy-induced thyroid dysfunction suggests that thyroid function should be carefully monitored in all patients receiving IL-2-based immunotherapy.