Study Objective: To assess the importance of the menstrual pattern as a marker for clinical and laboratory alterations related to metabolic syndrome (MS) and polycystic ovary syndrome (PCOS) among Brazilian adolescents. Design: A cross-sectional study. Setting: Endocrine Gynecology Outpatient Clinic of the Adolescent Health Studies Center (NESA) at the Pedro Ernesto University Hospital. Participants: 59 girls (12-19 years old) were classified by their menstrual cycles as regular (n = 23) and irregular (n = 36). Intervention: None. Main Outcome Measures: Biochemical collections were made of peripheral blood after fasting for 12 hours, and the oral glucose tolerance test with 75 g of anhydrous glucose. Results: PCOS, MS, and the criteria for MS were significantly more frequent (P < .05) in the subgroup with irregular menstruation. Adolescents with irregular cycles presented a significant increase in waist circumference, glycemia 2 hours after oral glucose overload (2 h), fasting and 2-h insulin, HOMA-IR, and triglycerides. In contrast, the glucose/insulin ratio, quantitative insulin-sensitivity check index, and HDL serum levels were significantly lower among patients with irregular menstruation, compared to those with regular cycles. In the logistic regression, we noted that insulin 2 h >= 75 mu IU/mL (r = 1.90; P = .018), waist circumference > 95 cm (r = 2.21; P = .006) and diagnosis of PCOS (r = 1.93; P = .023) were significantly correlated to irregular cycles. Conclusions: We concluded that close observation of menstrual cycle patterns is an important tool for identifying adolescents at higher risk of developing PCOS and MS.