Between 1991 and 1997, the use of marijuana by young people increased dramatically. Increased sophistication in the selective breeding of marijuana plants has led to a substantial increase in the potency of street samples during the past two decades. Trends suggest that the low-dose, self-experimentation type of use typical of the 1960's may be giving way to the high-potency, high-reward pattern of compulsive marijuana use prevalent during the late 1990's. Regular use of marijuana has been associated with cardiovascular, pulmonary, reproductive, and immunologic consequences. The physiological effects of marijuana use include an accelerated heart rate and a minimal rise in blood pressure. These effects are transient and usually are not deleterious to the otherwise healthy adolescent. The immediate pulmonary effect of smoking marijuana is bronchodilation, although with long-term use, the smoked particles act as an irritant, causing bronchoconstriction and eventual airway obstruction. The chronic effects are similar to those of smoking tobacco, and there seems to be a relationship between smoking marijuana and neoplastic changes in the lungs. Regarding behavioral and cognitive consequences, persons under the influence of the drug show impaired problem-solving skills and difficulty in organizing thoughts and conversing. Other adverse consequences of marijuana use include interference with coordination; the ability to judge elapsed time, speed, and distance; the ability to track a moving object; and reaction time. Regular use of marijuana also exerts a negative effect on short-term memory, learning, and attention span. Further, marijuana use often precedes the use of other more dangerous drugs. Although all marijuana users should be counseled about the dangers of the drug and the illicit nature of its use, marijuana is an addictive drug and is capable of producing dependency. Marijuana-dependent teens should be offered treatment options rather than punishment for their illness. 58 references