Lin-28 and lin-28B are RNA-binding proteins which can block microRNA let-7 maturation and affect the differentiation and proliferation of embryonic stem cells. Lin-28 may also regulate the expression of insulin-like growth factor II (IGF-II). As one of the pluripotent factors involved in making induced pluripotent stem cells (iPS), lin-28 is considered a potential therapeutic target for cancer treatment. To further understand the role of lin-28 in cancer, we analysed the expression of lin-28 and its homologue lin-28B in tumour samples, and evaluated their associations with let-7a maturation, IGF-II expression, disease features and outcomes in 211 patients with primary epithelial ovarian cancer. The analysis showed that both lin-28 and lin-28B were positively correlated with primary and pre-let-7a-3; lin-28B, not lin-28, was inversely correlated with mature let-7a. A positive correlation was also observed between lin-28B and IGF-II expression, while no association was found between lin-28B and IGF-I or IGFBP-3. The study further demonstrated that lin-28B expression was associated with the risk of disease progression and death; patients with high lin-28B had shorter progression-free and overall survival than those with low lin-28B. These results seem to support the findings of recent in vitro experiments, showing that lin-28 blocks the process of let-7a maturation. Our study also suggests that lin-28B may promote ovarian cancer progression and serve as an unfavourable prognostic marker for the disease. The correlation between lin-28B and IGF-II indicates that the growth factor may mediate the effect of lin-28B on tumour growth.