By means of a questionnaire we studied the menstrual patterns of 502 adolescent girls aged 10--19 years in 1979. The incidence of occasional or consistent primary dysmenorrhea was 43%. The incidence of dysmenorrhea increased considerably with increasing age and the interval between the menarche and the onset of dysmenorrhea was less than one year in 72% of subjects investigated. The occurrence of severe dysmenorrhea resulting in absence from school varied between 8 and 18% in various age groups.