Cytogenetic and clinical data were obtained from 22 former illicitdrug-users and 22 controls. Subjects were grouped into four categories depending on main drug of choice: opiates, psychedelics, amphetamines, and barbiturates. No relationship or difference was ascertained between abnormal metaphases and drug type, time off drugs, years on drugs, or life style. Drug users showed a disparity in the chromosome damage of the bone marrow and peripheral blood, since their peripheral blood lymphocytic cells exhibited a statistically significant number of abnormal metaphases, while the bone marrow was essentially normal. Karyotype analyses of marrow and blood metaphases did not reveal clones of cells. Although the chromosome damage is statistically different (P < 0.001) between the former illicit-drug users and controls, there is no proof of direct causality between drugs, drug abuse, and damaged chromosomes.