There have been reports that mice and rats developed cancer associated with implanted microchips. However, the majority of these mice and rats were being used for cancer studies when the tumors were found, and the rat and mice strains used in the studies are known to be more likely to develop cancer. Tumors associated with microchips in two dogs and two cats have been reported, but in at least one dog and one cat the tumor could not be directly linked to the microchip itself (and may have been caused by something else). For more details on the studies, read the AVMA’s literature review on Microchipping of Animals.