The rate of aging was studied in 306 persons working at Chernobyl Atomic Power Station after the accident by means of integral and partial biological age assessment. An accelerated rate of aging was found in 81% of men and in 77% of women in comparison with a control random population sample of Kiev. Persons younger than 45 years appeared to be more vulnerable to radiation. The biological age of persons who worked in the contaminated zone immediately after the disaster exceeded the biological age in those who arrived in Chernobyl 4 months later. The biological age in the investigated persons exceeded its average populational value for 5 years (the integral biological and partial cardiopulmonary age) and for 11 years for the partial psychological age. These data may underlie the concept of radiation progeroid syndrome as the form of accelerated aging.