Problems complicating a full assessment of the effects from Chernobyl included official secrecy and falsification of medical records by the USSR for the first 3.5 years after the catastrophe and the lack of reliable medical statistics in Ukraine, Belarus, and Russia. Official data concerning the thousands of cleanup workers (Chernobyl liquidators) who worked to control the emissions are especially difficult to reconstruct. Using criteria demanded by the International Atomic Energy Agency (IAEA), the World Health Organization (WHO), and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) resulted in marked underestimates of the number of fatalities and the extent and degree of sickness among those exposed to radioactive fallout from Chernobyl. Data on exposures were absent or grossly inadequate, while mounting indications of adverse effects became more and more apparent. Using objective information collected by scientists in the affected areas--comparisons of morbidity and mortality in territories characterized by identical physiography, demography, and economy, which differed only in the levels and spectra of radioactive contamination--revealed significant abnormalities associated with irradiation, unrelated to age or sex (e.g., stable chromosomal aberrations), as well as other genetic and nongenetic pathologies.