Chromosomal fragile sites analyses were performed in peripheral lymphocytes of 37 patients with lymphoma and 16 patients with leukemia, and also of 50 healthy individuals as controls. The results were: 1) The rates of chromosomal aberration and frequency of expression of fragile sites in patients with lymphoma and leukemia were significantly higher than those of normal controls. 2) There was a statistical association between 21 of 44 fragile sites and specific cancer breakpoints in patients with lymphoma and this was also the case with 19 of 30 fragile sites and specific cancer breakpoints in patients with leukemia. 3) Concordance between fragile sites and location of oncogenes in the diseases was established. The possible important role of fragile sites in the pathogenesis of lymphoma and leukemia is discussed.