Background Cancer has long inspired fear, but the effect of fear is not well understood; it seems both to facilitate and to deter early diagnosis behaviours. To elucidate fear's behavioural effects, we systematically reviewed and synthesised qualitative literature to explore what people fear about cancer. Methods We searched Medline, Embase, PsycInfo, Web of Science, AnthroSource, and Anthrobase for studies on cancer fear in breast, cervical, and colorectal cancer screening and analysed 102 studies from 26 countries using thematic synthesis. Results Fears of cancer emanated from a core view of cancer as a vicious, unpredictable, and indestructible enemy, evoking fears about its proximity, the (lack of) strategies to keep it at bay, the personal and social implications of succumbing, and fear of dying from cancer. Conclusions This view of cancer as ‘an enemy’ reprises the media's ‘war on cancer’ theme and may affect the acceptance of cancer early detection and prevention messages, since cancer's characteristics influenced whether ‘fight’ or ‘flight’ was considered appropriate.