Thyroid dysfunction has been reported in patients with malignant disease treated with recombinant alpha interferon. Two cases of hypothyroidism in patients with chronic hepatitis C treated with recombinant alpha interferon are reported. In one patient, interferon induced hypothyroidism in the absence of pre-existing thyroid dysfunction and in the other it aggravated a pre-existing thyroid dysfunction. Both patients developed a severe, sustained hypothyroidism requiring thyroxine treatment for one year or more after stopping alpha interferon. Diagnosis of hypothyroidism during treatment can be difficult because of the common side effects of alpha interferon. Thyroid function should be assessed before and during alpha interferon therapy in patients with chronic hepatitis C.