PurposeWhether anti-TNF agents increase, malignancy risk remains debated. The potential problems with comparing the number of observed malignancies in anti-TNF treated cohorts to number of expected malignancies in national cancer registries could have been an important issue. National registries give expected number of malignancies per organ involved regardless of if this is a secondary or a higher malignancy while anti-TNF agents are not or are sparingly used in patients with an existing or history of a malignancy. This results in a biased standardized incidence ratio (SIR). We aimed to formally look at the presence and the magnitude of this bias in published work.