Interleukin-2 (IL-2) is frequently incorporated in antineoplastic therapy: While the effect of interferon on the thyroid has been extensively studied the impact of other cytokines on thyroid function is less well understood. We monitored the thyroid function in six patients who received IL-2 in combination with tumor necrosis factor-alpha (TNF) or alpha-Interferon (alpha IFN). Hyperthyroxinemia with suppressed TSH developed within the first four weeks of IL-2 administration; during this phase, there was no technetium or iodine uptake by the thyroid gland. During the following few weeks, serum thyroxine decreased and serum TSH rose, consistent with the development of primary hypothyroidism; during this phase, thyroidal isotope incorporation was normal. All hypothyroid patients received thyroxine replacement therapy upon documentation of hypothyroidism; in several cases thyroxine was successfully discontinued after 2-3 months. None of the patients had detectable antithyroidal antibodies and none experienced thyroid-related pain, although two patients developed thyroid enlargement. We conclude that IL-2 administration is associated with the development of transient, subacute, painless thyroiditis. The frequency and severity of this complication requires further elucidation through systematic, prospective study.