Background <p>Bipolar affective disorder (BPAD) affected individuals on pharmacotherapy have a positive correlation with polycystic ovary syndrome (PCOS) apart from other endocrine abnormalities. Well-defined large multilocular adnexal masses on ultrasound should prompt further evaluation. We presented an unusual case of large multiloculated polycystic ovaries mimicking an ovarian tumour in a young girl on long-term lithium therapy.</p> Case report <p>A 26-year-old unmarried female with a history of BPAD taking lithium for the last eight years presented with amenorrhea for eight months and weight gain. Hormonal evaluation confirmed mildly raised prolactin levels. Ultrasound of the pelvis revealed a 13&#xa0;cm × 4&#xa0;cm bilateral multiloculated polycystic ovary mimicking an enlarged ovarian mass. Ovarian tumour markers were within normal range. The patient was prescribed low-dose combined oral contraceptive pills (OCPs) for three months followed by complete resolution of the sonographic abnormalities.</p> Conclusions <p>Clinicians should bear in mind the association of PCOS and BPAD, especially with lithium and second-generation antipsychotics. The clinical presentation usually depends on the doses of drugs and the severity of the condition. This case is unusual as radiological findings revealed large multiloculated polycystic ovaries resembling ovarian tumours. Unnecessary surgical intervention would compromise the ovarian reserve in a young female.</p>