In a single-blind study chromosome analyses were performed on three groups of subjects: 1) nine heavy users of LSD-25 who have continued to use the drug; 2) eight heavy users of LSD-25 who have ceased to use it; and 3) eight drug-free control subjects. The results revealed no significant differences in chromosome break-age rates between any of the three groups and therefore no temporal factor was elicited regarding chromosome damage and LSD ingestion. The mean breakage rates for all three groups were 1.8 percent or less and did not support earlier findings that LSD ingestion causes an increase in chromosome aberrations over those observed in a normal control population. The authors emphasize the need for a more controlled longitudinal study.Abstract Teaser