Fragile sites are non-staining gaps and breaks in specific points of chromosomes. These sites also include acentric fragments, triradial figures and several rearrangements. Although this issue has been controversial recently, they may be related to structural chromosomal rearrangement in some neoplasms. In this study, the expression of fragile sites induced by aphidicolin (Apc), 5-bromodeoxyuridine (BrJU) and caffeine was investigated on prometaphase chromosomes obtained from the peripheral blood lymphocytes of 36 patients with rectum cancer, 30 first-degree relatives and 30 normal healthy controls. The results of the structural chromosome aberrations determined in patients and their first-degree relatives were significantly higher than those in control subjects (P < 0.001). We determined aphidicolin type common fragile sites (1p36, 1p31, 1p21, 1q21, 1q25, 1q44, 2p24, 2q21, 2q33, 2q37, 3p14, 5q21, 5q33, 13q13, 14q24, 16q23 and 18q21). When the rates of sites such as 1p21, 1q25, 2q33, 3p14, 5q21 and 14q24 in patients and in their first-degree relatives were compared with the control group, the difference was statistically significant. Our results indicated an increased genetic instability in patients with rectum cancer and their first degree relatives. Therefore, the increase of fragile site expression may be an important marker showing genetic predisposition to rectum cancer. (C) 2000 Elsevier Science ireland Ltd. All rights reserved.