During the 13-year period subsequent to the Chernobyl accident, the morbidity of malignant renal tumors in Ukraine has increased from 4. 7 to 7.5 per 100,000 of total population. Cesium 137 ((137)Cs) accounts for 90% of the incorporated radioactivity in the Ukrainian population, which has been exposed to long-term, low dose ionizing radiation and 90% of the more labile pool of ((137)Cs) is excreted via kidneys. The present study was performed to evaluate the histopathological features and the immunohistochemical status of proliferating cell nuclear antigen (PCNA) and K-ras in renal cell carcinomas (RCCs) of 236 Ukrainian patients (groups I to V), which represents a varying degrees of internal exposure to radiation and were operated in 2 different periods of time after the Chernobyl accident. The control group VI of 112 analog patients with RCCs was selected in Spain. The strong significant differences between the Ukrainian and Spanish groups were found in tumoral nuclear grade, in the percentage of sarcomatoid changes, the level of the peritumoral inflammatory response as well as in the peritumoral lesions. The dramatic increase of aggressivity and proliferative activity supported by strong PCNA and K-ras expression of RCCs from Ukrainian groups, associated with chronic radiation nephropathy of peritumoral kidney tissue, showed good correlation with the duration of radiation exposure and confirmed the influence of chronic but regular and sustained low dose of ionizing radiation on renal carcinogenesis of the Ukrainian population.