The frequency and distribution of aphidicolin induced common fragile sites was evaluated on chromosomes of peripheral blood lymphocytes in 10 breast and 10 colorectal cancer patients, and 10 healthy controls to determine correlation between specific fragile sites and cancer breakpoints. Fifty complete metaphases were screened from each culture and the results were evaluated by Student's t-test. The total number of fragile sites was found as 933 in breast cancer patients, 950 in colorectal cancer patients and 501 in control group. Both the number of aberrations per cell and number of aberrations per damaged cell were significantly higher in the patient groups. These findings indicate that genetic instability in the breast and colorectal cancer patients increased and fragile sites may play a critical role in the pathogenesis of breast and colorectal cancer.