Peripheral blood leukocytes from 56 users of psychoactive drugs and 16 non-drug-users were cultured and examined for the presence of structural chromosomal abnormalities. None of the subjects admitted recent exposure to x-rays, other irradiation, or viral infection. Drugusers were divided into five groups: those who smoked marihuana lightly; psychiatric patients treated with phenothiazine; and one group each of heavy users of two or more combinations of marihuana, heroin, and amphetamine. With the exception of the controls and the light users of marihuana, all groups showed elevated incidences of chromosomal aberrations. Increases were not general in any one group, but were largely accounted for by a few individuals within each group with more than one aberration each. Although it may be that any or all of the drugs could damage chromosomes in this way, it seems more likely that some other factor or factors common to drug-users might be responsible.