Background Polycystic ovary syndrome (PCOS) is considered to be the most common endocrine disorder in women of reproductive age.The involvement of an abnormal menstrual cycle in the etiology of PCOS remains unclear.We aimed to analyze the characteristics of abnormal menstrual cycles and their association with PCOS in community and hospital patient populations.Methods Women with PCOS identified from 2111 permanent female residents in the community of Beijing and 506 outpatients obtained from the reproductive clinic of Peking University Third Hospital were recruited for this study,comprising the PCOS community group and the PCOS hospital group, respectively.Each group was further divided into four subgroups according to the length of menstrual cycles: ＜21 days; 21-34 days; 35-60 days; and ＞60 days.Women in each group were interviewed using a questionnaire to assess factors including age, age of menarche, menstrual cycle history, related family history, and modified Ferriman-Gallwey (mF-G) score.All women received transvaginal ultrasound scan and had fasting blood samples taken for endocrine evaluation.A two-tailed P value of ＜0.05 was considered significant.Results In the community population, the prevalence of abnormal menstrual cycle was 27.19% (574/2111).The prevalence of PCOS in the community was 6.11% (129/2111) according to Rotterdam criteria.In the community group,the most common menstrual cycle length was 35-60 days, whereas for the hospital group, it was ＞60 days.In both the community and hospital groups, the most common phenotype of PCOS was that of Oligo/amenorrhea+PCO+hyperandrogenism (HA) (O+P+H) (P=0.000).With increasing cycle length of 35-60 days to ＞60 days, the percentages of Oligo/amenorrhea+PCO (O+P) and O+P+H were found to significantly decrease in the community group and significantly increase in the hospital group (P=0.000 for each).In the hospital group, as the menstrual cycle length increased from 35-60 days to ＞60 days, the rate of spontaneous abortion increased significantly (P=0.000), meanwhile the rate of poorly-secreted endometrium and abnormal endometrial hyperplasia increased significantly (P=0.000).Conclusions The prevalence of PCOS in the Beijing community of women was 6.11%.Oligo/enorrhea was the most common type of abnormal menstrual cycle and may be an indicator for PCOS and endometrial lesions.Gynecologists should seek relevant medical information from women in the community to promptly diagnose PCOS and then follow up patients for potential development of subsequent complications.