The possible involvement of mycotoxins in chromosomal alterations in patients with Balkan endemic nephropathy (BEN) was investigated cytogenetically. Lymphocyte cultures from patients with BEN and from individuals from a nonendemic region were examined and compared with cultures from healthy people which had been incubated in vitro with noncytotoxic doses of ochratoxin A. Significantly increased numbers of various numerical and structural anomalies were found in patients with BEN. Chromosome X in female patients occurred in both monosomic and polysomic forms. A 'prosomization' effect was seen along the entire length of supernumerary X chromosomes, manifested by retarded contraction resembling early mitotic stages, with a comparably detailed band pattern. No other specific numerical or structural change was found consistently in BEN patients. Incubation of the lymphocytes of healthy people with ochratoxin A induced similar aberrations and prosomization. These findings may support the hypothesis that ochratoxin A is involved in the pathogenesis of BEN.