Background & aim: Primary dysmenorrhea is a common problem among women, resulting in a decline in their performance level and quality of life. Previous studies have not indicated a definite relationship between body mass index (BMI) and dysmenorrhea. Therefore, this study aimed to determine the severity of dysmenorrhea and investigate its relationship with BMI among female adolescents in Hamadan, Iran. Methods:This cross-sectional study was conducted on 579 single, female adolescents in schools of Hamadan, Iran in 2011. Students with primary dysmenorrhea (without any other conditions) were selected, using random cluster sampling. Demographic characteristics and menstrual cycle information were collected, using two separate questionnaires. BMI was recorded in all participants and visual analogue scale was applied for pain assessment. Chi-square, t-test, Fisher's exact test and logistic regression were used to examine the relationship between variables. Data were analyzed, using SPSS version 16. P-value less than 0.05 was considered statistically significant. Results: The mean age, BMI and age at menarche in participants were 15.94±1.17 years, 21.16±3.36 kg/m2 and 12.92± 1.05 years, respectively. The frequency of dysmenorrhea was estimated to be 85.31%. The frequency and severity of dysmenorrhea was higher in the normal-weight group than other subjects. No relationship was seen between the severity and duration of dysmenorrhea with BMI. But dysmenorrhea had a significant relationship with age at menarche and premenstrual syndrome (P