Background: Polycystic ovarian syndrome (PCOS) is the most common endocrine cause of infertility affecting about 1 in 10 women of reproductive age. This study determined the proportion of infertile women with PCOS and compared clinical and sonographic features in infertile women with PCOS and without PCOS.Methods: This was a prospective comparative cross-sectional study at the Lagos State University Teaching Hospital over a 6 month period. One hundred and fifty two infertile women were recruited and had trans-vaginal ultrasonography for the presence and absence of polycystic ovaries. The diagnosis of PCOS was made using the Rotterdam criteria with other relevant socio-demographic and clinical data noted in the study proforma. Student t and chi-square tests were used as appropriate.Results: The prevalence of PCOS among infertile women was 32.9%. The mean age of infertile women diagnosed with PCOS and without PCOS was 33±4.90 and 32.71±5.00 years, respectively. Women with PCOS had significantly higher BMI than women without PCOS. Increased ovarian volume, peripheral follicular distribution and increased stromal echogenicity were observed in 60%, 74% and 94% of women with PCOS on trans-vaginal ultrasound compared to 11.8%, 2% and 2% of women without PCOS, respectively.Conclusions: The prevalence of PCOS among infertile women is high. Clinical characteristics of women with PCOS and women without PCOS are similar except in their BMI and features that constitute the diagnostic criteria for PCOS.