Data on the excretion of Tamm-Horsfall protein (THP) in subjects living in an area of Balkan endemic nephropathy (BEN) are reported. The study subjects were divided into groups as follows: diseased, suspect, "at risk" and others, according to previously adopted criteria. The THP excretion in "at risk" subjects was found to be significantly higher as compared to control subjects. The difference between these two groups could not be registered by any other clinical or laboratory diagnostic methods. No difference in the excretion of THP was observed between the groups of others and control subjects. According to the results obtained, the excretion of THP may be considered a possibly useful additional diagnostic test for the detection of subjects with the latent, early subclinical phase of BEN. On the other hand, the data obtained shed some more light on the still obscure pathogenesis and natural history of BEN.