The association of Hashimoto's thyroiditis (HT) with papillary thyroid cancer (PTC) has been still under debate. Some suggest that these two are positively correlated, whereas other studies report no relationship. We performed a systematic literature review of original studies to investigate the correlation between HT and PTC. The existing data provide inconsistent evidence favoring a causal relationship between HT and PTC. The average prevalence rate of PTC in patients with HT was 1.2% in 8 fine needle aspiration (FNA) studies of 18,023 specimens and 27.6% in 8 archival thyroidectomy studies of 9884 specimens. The risk ratio of PTC in HT specimens ranged from 0.39 to 1.00 in the FNA group (average RR 0.69) in contrast to 1.15 to 4.16 from thyroidectomy studies (average RR 1.59). Population-based fine needle aspiration biopsy studies report no relationship, whereas many of the studies using thyroidectomy specimens report a positive relationship, possibly related to selection bias. Several studies identified a few biomolecular markers, including the PI3K/Akt pathway, RET/PTC gene rearrangements, p63 protein, and loss of heterozygosity of hOGG1, that are potentially involved in neoplastic transformation from HT to PTC. So far, no causal genetic linkage has been confirmed. PTC with concurrent HT is associated with female gender, young age, less aggressive disease such as small tumor size, less frequent capsular invasion and nodal metastasis, and better outcome. However, more prospective studies with long term follow-up are needed to further elucidate this relationship and prognosis. Careful observation and follow-up of HT patients is recommended, especially those with nodular variants.