What is the risk of cancer from a ct scan?

Extremely low.: The amount of radiation a patient is exposed to with a single computerized tomography (ct) scan is way below the amount required to inflict the necessary dna damage to cause cancer. In fact, studies have proven that a pregnant woman may safely receive a single ct with no negative effects to the fetus (although efforts should still be made to minimize unnecessary exposure).

Small but real: Repeat exposure increases risk of cancer due to radiation. The risk is small but real, and depends on frequency of exposure and your individual predisposition to radiation damage.

...and more: I'll add to what they said. Everything we know about risk from radiation is extrapolated from large exposures like hiroshima etc. This extrapolation requires the assumption that dna repair capacity after a small dose of radiation is the same as from a large exposure. This assumption is not accurate, and therefore, estimates of risk from small exposures (e.g. A ct) are probably greatly exaggerated.