Exposure to ionizing radiations (Rx) has been implicated as a causative factor of chronic myelogenous leukemia (CML). We performed a retrospective study of 443 consecutive CML patients, looking for a history of significant exposure to Rx, and evaluated the clinical and hematological characteristics in order to find any difference between radiation-related CML patients and those with de novo CML. We identified 406 patients without known exposure to mutagens (group I) and 37 patients with prior significant exposure to Rx (group II). In comparison to patients of group I, those of group II showed particular clinical and hematological features: significantly lower incidence of bulky splenomegaly (p < 0.05) and hyperleukocytosis (WBC > 100 x 10(9)/l; p < 0.05); significantly higher incidence of anemia (Hb < 10 g/dl; p < 0.01). Patients with radiation-related CML had a significantly better survival than those with de novo CML (median survival 61 months vs 42 months; p < 0.05). In conclusion, this study of a large cohort of CML patients indicates that the subgroup of patients with a history of significant exposure to ionizing radiation has particular clinical and hematological features at onset (lower tumor burden, higher frequency of anemia) and a better survival.