The authors refer to the risks of using cannabis among pupils of 8th and 9th grade of basic schools and students of 3rd and 4th grades of eight-year grammar schools in Prague and České Budějovice. 17.3% of 1431 adolescents at the average age of 14.5 years reported an experience with marijuana or hashish during last year. Between the age of 14 and 15, an experience with cannabis increased from 12% to 22%. 7.7% reported regular (once a month and more often) use of cannabis. The authors found a higher prevalence of cannabis use among Prague adolescents; no signifi cant difference was found between boys and girls. In comparison with their peers users of cannabis spent more often their leisure time in a bunch, they went out for entertainment in the evening more often, they had higher pocket money, they spent less time with their parents, they had confl icting relationship with one of their parents, more often they reported that one of their parents had problems with alcohol. They smoked, drank alcohol and had an experience with other drugs more often. In Windle’s questionnaire showed higher score in a scale of attention disorder and hyperactivity, opposing disorder and behaviour disorders. Also a higher score of anomie and depression was found in them.

        Key words: addictions, cannabis, behaviour disorders, adolescence.