BACKGROUND: Hirsutism is the condition of excessive terminal hair growth in women with a typical male pattern distribution. Hirsutism is a common disorder that affects about 5% -10% of women of reproductive age. Adipose tissue contributes up to 50% of the circulating testosterone in premenopausal women Because of excessive androgen production in fat tissue. Therefore, it seems that hirsutism must be more common in people with simple obesity but controversy exist regarding this subject. The aim of this study is to evaluate the relation between Body Mass Index and hirsutism in a representative sample of Iranian woman. METHODS: This is a cross sectional case control clinical trial. The study involved 800 individuals; 400 hirsute females and 400 healthy women as control group. The mean age of the participants was 28 ± 6.2 years. Hirsutism was determined by the Ferriman-Gallwey scoring system. Height and weight were measured by a Seca scale, Body Mass Index was calculated as weight/height² (kg/m²), and collected data were analyzed by SPSS software version 18 using T-test and chi-square statistical test. RESULTS: There were no significant differences between the two groups regarding age and height. However, Body Mass Index and weight were significantly higher in the case group than the control group. The chi square test revealed significant differences between the case and control groups regarding Body Mass Index (P < 0.001). CONCLUSION: In the current study hirsutism was more common in patients with a higher Body Mass Index. The increased frequency of hirsutism in overweight women could be explained by increased insulin resistance and more androgen production by adipose tissue. Keywords: Body Mass Index, Hirsutism, Obesity.