Introduction: Obesity is a major factor in development of insulin resistance and metabolic features in polycystic ovary syndrome (PCOS) patients. Nearly two-thirds of Indian patients with PCOS are lean (30 of 37 confirmed cases of PCOS in our previous community prevalence study were lean), in contrast to the Caucasians. Metabolic parameters insulin resistance and beta cell function have not been characterized well in this group of lean PCOS. Objective: To study the metabolic features, beta cell function, and insulin resistance in lean PCOS patients attending the endocrine clinic. Materials and Methods: Twenty lean, (body mass index (BMI) <23 kg/mµ2) cases of PCOS (diagnosed by Rotterdam criteria) were compared with 44 obese PCOS cases, age 42 years, and BMI comparable controls and 31 obese controls. All subjects underwent detailed history, examination, and 2 h oral glucose tolerance test (OGTT). Various indices were calculated from the glucose and insulin values measured in OGTT, beta cell function was also assessed by measuring fasting C-peptide. Result: Waist-hip ratio, post glucose blood glucose (PPG), and fasting glucose to insulin ration (FGIR) were different in lean cases and controls but other parameters; HOMA-IR, c-peptide, and prevalence of metabolic syndrome (International Diabetes Federation (IDF) criteria) were similar. Compared with obese PCOS, lean PCOS cases had less prevalence of hirsutism, acanthosis, and metabolic syndrome; low fasting glucose, PPG, and FGIR; but higher luteinizing hormone (LH). Serum alanine aminotransferase (ALT) was significantly higher in obese PCOS cases than obese controls. Fasting C-peptide and homeostasis model assessment (HOMA)-B were comparable in all groups of patients and controls. Conclusion: Metabolic characteristics and beta cell function in lean PCOS patients is similar to obese PCOS. Drawback of the study is the small sample size of lean patients and a hospital based study, which could mean patients who were more symptomatic were enrolled. [ABSTRACT FROM AUTHOR]