OBJECTIVE. To study the presentations, diagnoses, and outcomes in adolescents with menstrual disorders. DESIGN. Prospective cohort study. SETTING. Paediatric and Adolescent Gynaecology Clinic, Hong Kong. PARTICIPANTS. A total of 577 adolescents aged 14 to 19 years. MAIN OUTCOME MEASURES. The presentations and diagnoses of adolescents with menstrual disorders were reviewed and their menstrual outcomes determined by a telephone survey. RESULTS. In all, 47% presented with menorrhagia, prolonged menstruation, and short menstrual cycles; 27% had secondary amenorrhoea, 12% had dysmenorrhoea, 11% had oligomenorrhoea, and 3% had primary amenorrhoea. Significant diagnoses included congenital genital tract anomalies, premature ovarian failure, anorexia nervosa, and polycystic ovarian syndrome. Polycystic ovarian syndrome was diagnosed in 16% of the cohort. In all, 24% of these 577 patients had abnormal menstrual cycles 4 years later. Direct logistic regression analysis indicated a cycle length of more than 35 days at presentation (adjusted odds ratio=2.8; 95% confidence interval, 1.8-4.5), previous diagnosis of polycystic ovarian syndrome (adjusted odds ratio=2.0; 95% confidence interval, 1.1-3.4), and current body mass index of 23 kg/m(2) or higher (adjusted odds ratio=1.8; 95% confidence interval, 1.0-3.0) were risk factors for persistently long menstrual cycle exceeding 35 days. Adolescents who were screened out with a definitive diagnosis after initial assessment were at low risk of persistently long menstrual cycles at follow-up (adjusted odds ratio=0.3; 95% confidence interval, 0.1-0.8). CONCLUSIONS. Adolescent menstrual disorders should not be ignored. Long cycle, diagnosis of polycystic ovarian syndrome at first consultation, and a current body mass index of 23 kg/m(2) or higher were statistically associated with persistent problems.