Chronic enzootic haematuria (CEH) is a severe syndrome due to prolonged ingestion of toxic principles of bracken fern, such as quercetin and ptaquiloside. Little information is available on chromosomal instability of cattle with access to bracken fern and suffering from CEH. In the present study, 45 cattle, aged from 7 to 12 years and pastured in the south of Italy, were cytogenetically investigated for the first time in search of both chromosomal aberrations (aneuploidy, gaps, chromatid breaks, chromosome breaks and fragments) and sister chromatid exchanges (SCEs). Of these animals, 30 (group 1) had access to bracken fern and showed signs of CEH, and 15 (group 2; control) did not. Percentage of abnormal cells (aneuploidy, chromatid breaks, chromosome breaks and fragments) was higher in animals affected by CEH (34.7%, group 1) than that (24.3%) reached in the control (group 2). The same results were achieved when including gaps. Indeed, the mean number of cells with structural aberrations excluding gaps (chromatid breaks, chromosome breaks and fragments) per cell was higher (P<0.001) in animals affected by CEH (0.16+/-0.36) than that (0.09+/-0.29) found in the control. Chromosome fragility in cells of animals affected by CEH was also confirmed when applying the SCE test: statistically higher levels (P<0.001) of SCEs were observed in animals with CEH (7.35+/-3.59 SCE/cell, group 1) than those in the control (5.40+/-2.68 SCE/cell).