We have investigated the prevalence of proteinuria in patients with Graves' disease and chronic autoimmune thyroiditis attending a routine thyroid clinic. Using the urine protein creatinine index, proteinuria was found in 29.8% of patients with autoimmune thyroid disease and in 9.5% of patients attending the same clinic but without these conditions. When patients with Graves' disease were treated with 131I, proteinuria measured by 24 h collections developed in 9 of 14 patients without pre-existing proteinuria and appeared to diminish in 4 patients in whom proteinuria had been present before treatment. The prevalence and fluctuation of proteinuria was independent of thyroglobulin and microsomal antibody levels. We were unable to confirm previous reports of a high prevalence of circulating immune complexes in autoimmune thyroid disease; complexes were detected in only 7.9% of patients and did not correlate with proteinuria. The causes of mild proteinuria in autoimmune thyroid disease are not apparent, but previous case reports suggesting that membranous glomerulonephritis is associated with Graves' disease, albeit rarely, indicate that immunological mechanisms may be implicated.