Crack produces rapid and striking stimulant effects when smoked. Crack is used by a small minority of adult and student populations but by a large proportion of cocaine users and groups with heavy drug use. Use does not appear to be increasing in general populations, but no trend studies exist 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, and pulmonary and cardiac abnormalities. In addition, 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 rooms filled with crack smoke. Improved treatment and prevention programs for crack use are needed. Tables and 40 references (Author abstract modified)