This study is the first cytogenetic investigation of healthy relatives of patients with Balkan endemic nephropathy (BEN) who were born in nonendemic areas. Characteristics of BEN No. 3 chromosomal anomalies (extremely high frequencies of 3q25 homologue discordance - 68.5 +/- (SD) 5.03% vs. 6.65 +/- 0.95% in controls, p < 0.001; chromosome breaks at 3q25 band - 0.79 +/- 0.25% vs. 0.01% in controls, p < 0.001; structural aberrations affecting 3q25 band), very high frequency of acquired chromosomal aberrations (5.74 +/- 0.64% vs. 1.72 +/- 0.3% in controls) and a family history with 1 or 2 BEN parents were identified in 5 relatives. It is proposed that they are at high risk for developing the disease and that a genetic mechanism might be involved in the etiology of BEN.