Objective Although menstrual irregularity is associated with insulin resistance and hyperandrogenism, the relationship between the severity of menstrual infrequency and clinical phenotypes in young women with oligomenorrhoea (OM) is unclear. We evaluated whether a longer menstrual cycle length is associated with less favourable metabolic features. Design/Patients/Measurements A total of 1174 young women (aged 19-39 years) with a menstrual cycle length over 40 days and 1430 women with regular menstrual cycles participated voluntarily. Metabolic parameters, insulin sensitivity index (ISI) and testosterone were measured. Oligomenorrhoeic women were divided into three groups: (i) polycystic ovary syndrome (PCOS) by National Institute of Health criteria, (ii) severe OM (menstrual cycle length >60 days), and (iii) mild OM (menstrual cycle length 40-60 days). Results In normal-weight women (BMI < 23 kg/m2), the degrees of insulin resistance and hyperandrogenaemia are the highest in PCOS and higher in severe OM compared with mild OM. In overweight or obese women, PCOS was more insulin resistant and hyperandrogenaemic, but there was no difference between severe and mild OM. After excluding PCOS, women with severe OM showed a twofold increased risk of metabolic syndrome compared with regular cycling women (odds ratio 2·4, 95% confidence interval 1·1-5·6). By linear regression analysis, a longer menstrual cycle length was associated with ISI after adjustment for age, BMI, metabolic risk factors and testosterone. Conclusions Women with a menstrual cycle length over 60 days should be more closely monitored for the metabolic syndrome than women with a menstrual cycle length of 40-60 days, even if they have no PCOS.