Context: The knowledge of normal variation of reproductive hormones, internal genitalia imaging, and the prevalence of gynecological disorders in adolescent girls is limited. Objective: The study aimed to describe reproductive parameters in postmenarchal girls from the general population including the frequency of oligomenorrhea, polycystic ovary syndrome, and use of hormonal contraception. Design: The Copenhagen Mother-Child Cohort is a population-based longitudinal birth cohort of 1210 girls born between 1997 and 2002. Setting: University hospital. Participants: A total of 317 girls were included, with a median age of 16.1 years and time since menarche of 2.9 years. Main Outcome Measure(s): Tanner stage, height, weight, age at menarche, menstrual cycle length and regularity, ovarian/uterine volume, and number of follicles were recorded. Serum concentrations of FSH, LH, anti-Müllerian hormone (AMH), inhibin B, estradiol, testosterone, SHBG, androstenedione, dehydroepiandrosterone sulfate, 17-OH-progesterone, and IGF-1 were measured. Results: Twenty girls (6.3%) had oligomenorrhea and differed significantly in serum androgens and AMH, age at and time since menarche from girls with regular cycles. Twenty-seven girls were classified with PCOS (8.5%) and had significantly higher 17-OH-progesterone, estradiol, AMH, LH, and age at menarche than the reference group. Girls on oral contraception had significantly higher serum SHBG concentrations and lower serum concentrations of all hormones except AMH and IGF-1. Ovarian follicles 2 to 29.9 mm correlated positively with serum AMH (P < 0.0001). Conclusions: Most 16-year-old girls had regular menstrual cycles, normal reproductive hormones, and uterine and ovarian ultrasound. Serum AMH reflected ovarian follicle count and may be a useful biomarker of ovarian reserve.