It is well recognized that LSD (lysergide) can give serious adverse reactions, including suicide and prolonged psychosis; Smart and Bateman (1967) have reviewed the subject thoroughly. However, examples reported refer largely to cases where the LSD was self-administered. Medical case reports usually cover small case numbers, and publication may be determined by high adverse reaction rates. There has been no methodical survey of the pooled experience of psychiatrists since Cohen's study in 1960. This reported 44 replies sent out to 62 American investigators who had published papers or whose work was known to the author. Replies covered 5,000 subjects with 25,000 LSD or mescaline sessions. In this series, there were only two suicides that Cohen regarded as directly related to the LSD, and there were psychotic reactions (lasting more than 48 hours) at a rate of 0·8 per 1,000 experimental subjects and 1·8 per 1000 patients. It is unfortunate that one-third of the investigators failed to reply to Cohen's questionnaire.