This chapter examines the evidence for ethnic variation in the polycystic ovary syndrome (PCOS) phenotype and explores the possible basis of this phenomenon. Evaluating ethnic variations in the expression of PCOS requires systematic review of the strength of the evidence, preferably from population-based data or from large samples in the clinic setting. The chapter evaluates reports of differing expression of hyperandrogenism, obesity, insulin resistance and metabolic manifestations. The reports discussed mainly comprise comparative and case-control studies providing level III evidence. The available data show variations in the PCOS phenotype among Caribbean Hispanic, Mexican American, Japanese, indigenous Chinese/Taiwanese, migrant versus indigenous South Asian, Thai, Malay, southern European, indigenous Canadian and migrant Arab women. Reports on the degree of hyperandrogenism in PCOS in women from different regions of the world are varied, as they are based on various diagnostic criteria, clinical settings, age and ethnic origins.