Crack is a potent form of cocaine which results in rapid and striking stimulant effects when smoked. This paper reviews epidemiological research on the extent of use as well as reports of adverse effects. Crack is used by a small minority of adult and student populations but by a large proportion of cocaine users and heavy drug-using groups. Use does not appear to be increasing in general populations, but there are no trend studies for high-risk groups. Crack users tend to be young, heavy polydrug users, many of whom have serious drug abuse problems. The adverse reactions to crack are similar to those of cocaine and include effects on offspring, neurological and psychiatric problems, as well as pulmonary and cardiac abnormalities. However, two adverse reactions unique to crack have been reported. One relates to lung infiltrates and bronchospasm. The other involves neurological symptoms among children living in crack smoke-filled rooms. There is a need for improved treatment and preventive programs for crack use.