Human foamy virus (HFV) is a recently characterized member of the retrovirus subfamily Spumaretrovirinae. HFV has a complex structure: it encodes the classical three retroviral structural genes gag, pol and env, but also possesses additional regulatory, so-called bel sequences. Foamy viruses have been discussed occasionally as being possibly involved in the pathogenesis of autoimmune thyroiditis. By indirect immunofluorescence we were able to demonstrate the expression of HFV gag proteins on the epithelial cells of 7/7 thyroid glands of patients with Graves' disease, but not in those of 9 patients with struma parenchymatosa, 3 with follicular carcinoma and 2 normal thyroids. The thyroids of 5 patients with Hashimoto's disease were also negative with the exception of a single small focus in 1 case. These observations may have importance for our understanding of the development of Graves' disease.