This is a comparative cross – sectional prospective study with the aim of evaluation of prevalence and correlation between menstrual irregularity and PCO in postmenarcheal age. This evaluation has done in two groups of healthy adolescent girls 12-19 years old who admitted in shahid Mostafa Khomeini hospital, IRIB Clinic and a private clinic. We compared 47 adolescent girls who had irregular menstruation (as study group) with 22 adolescent girls who had normal menses (as control group) We did abdominal ultrasonography (US) in both groups and evaluated US features of PCO on them .This research shows that PCO was seen to be more common in adolescent girls with menstrual irregularity (42/5% vs 13.6% , p<0.011) with a tendency to be more frequent in oligomenorrhea–amenorrhea group than in polymenorrhea (51.6%vs 25% P>0.08).We have also seen increased LH/FSH in 86.9% and abnormal androgens profiles in 69.5% in girls with PCO. These results suggest that US changes compatible with PCO is a frequent finding in adolescent girls with menstrual disturbance preferably in those with oligoamenorreha