The use of cannabis and other illicit drugs has become a widespread problem among youth in many industrialized countries. As such, it is important to understand the ramifications of such substance abuse. The author administered 2 questionnaires to 7,577 men 18 years old, who were conscripted for military service in Sweden in 1969 and 1970. The first questionnaire focused on social and family background, psychological health, and physical health. The second questionnaire focused on alcohol consumption patterns, substance use, and tobacco use. The 7,577 subjects were followed through the age of 45 with regard to their alcohol and drug use. Data from the first two questionnaires were compared to the Injection Mark Study and the Stockholm County Council in-patient register and cause-of-death register to ascertain adult alcohol and drug use. The comparison revealed that 3.8 percent of the 7,577 subjects had either been treated for or had died of complications from adult alcohol consumption. Another 2.2 percent had either been treated for or had died of adult drug abuse. Results of bivariate statistical analysis indicated that a combination of adolescent cannabis and alcohol abuse was associated with both adult alcohol abuse and drug abuse. The effects of adolescent alcohol use alone or cannabis use alone were not as strongly predictive of adult substance abuse problems. Results of multivariate analysis showed that tobacco smoking was associated with adult alcohol abuse. The author thus concluded that the combined use of cannabis and alcohol during adolescence presents a greater risk factor for adult alcohol and drug abuse than either adolescent cannabis use or alcohol use alone. Results have treatment implications for targeting adolescents at most risk because of their multi-drug use. Tables, references