Thyroid dysfunction is a well-recognised side-effect of treatment with interleukin-2 (IL2). We assessed the correlation between the development of abnormal thyroid function and tumour response in 13 patients receiving IL2 and interferon-2 alpha (IFN2 alpha) for advanced malignancy. Seven patients had normal thyroid function during treatment, and all of these patients have since died of progressive disease. Of six patients who did develop thyroid dysfunction during treatment, one patient has died of progressive disease. However, statistically we were unable to confirm a definite correlation between the development of thyroid dysfunction and survival in this small group of patients.