The G band characteristic of chromosomes in 15 patients with Balkan endemic nephropathy (BEN), 20 relatives with no features of the disease and 25 healthy controls were investigated by cytogenetic analysis of lymphocyte cultures from peripheral blood. A specific chromosome marker in BEN was established in one No. 3 chromosome homologue characterized by a discordance in the banding patterns of the long arm, shortening the band 3q25 with faster fusion of subbands q26.1 and q26.3, and lack of differentiation of q24. Among the patients there were a mosaic case with a caryotype: 46,XY/46,XY,t(1;3) (q11.2;q25) and another one with a t(2;3)(p11.2; q25) in a tetraploid lymphocyte cell. The evidence of shortening of q25 and postzygotic mutations for 1/3 translocation and 2/3 translocation with the breakpoints on chromosome 3 being in 3q25 suggested that the critical band for BEN is 3q25.