Background: Hirsutism is a common clinical problem with different etiologic factors. Hence, clinical and laboratory studies are necessary to find these factors. Objectives: To determine the etiologic factors of hirsutism with clinical and paraclinical studies including TSH, DHEA-S, LH/FSH, testosterone, prolactin, 17-OH progesterone and sonography of ovary. Patients and Methods: This cross sectional study evaluated clinical and paraclinical findings of 110 hirsute patients who referred to the dermatology clinic of Tehran Razi Hospital during a period of one and half years, in 2000-2001. Results: Polycystic ovary syndrome (PCOs) was diagnosed in 54 patients (49%) including 14 cases (26%) with regular menstruation. 43 patients (39%) had laboratory findings compatible with PCOs and LH/FSH>3 was the most valuable one in 41 patients (37%). 53 patients (48%) with regular menstrual cycles and normal laboratory and sonographic findings had idiopathic hirsutism. Late onset adrenal hyperplasia due to partial 21-hydroxylase deficiency was diagnosed in two cases (1.8%). In one patient with irregular menstruation, pituitary prolactinoma was confirmed by brain imaging techniques. Conclusion: Routine laboratory evaluations are mandatory in the management of hirsutism, regardless of menstruation cycles.