Two-hundred and seventeen subjects underwent transvaginal ultrasound;-17 (8%) had unilateral polycystic ovary (PCO). Twelve percent of subjects with unilateral scan-PCO had oligomenorrhea, 24% were amenorrhea, 23% were hirsute and 29% had acne. Biochemical parameters were compared between subjects with unilateral scan-PCO and those with bilateral scan-PCO (n=200) as well as a group of scan-normal women (n=29). No significant difference was found between subjects with bilateral and unilateral scan-PCO suggesting that these 2 groups are biochemically similar. The existence of unilateral scan-PCO suggests that PCOS may be a primary ovarian disorder.