West Nile virus is becoming increasingly prevalent in the USA, causing fever, encephalitis, meningitis and many fatalities. Spread of the disease is reduced by controlling the mosquito vectors by a variety of means, including the use of pyrethroid insecticides, which are currently under scrutiny for potential carcinogenic effects in humans. Pyrethrins and resmethrin, a pyrethroid, have been shown to cause tumours in rat and mouse models respectively. However, the tumours appear to be caused by liver enzyme induction and hypertrophy rather than genotoxicity, and the results are therefore unlikely to be applicable to humans. Nonetheless, for resmethrin, the US Environmental Protection Agency (EPA) has concluded that there is a likely risk of carcinogenicity in humans, requiring the manufacturers to provide more detailed data to prove that it can be used safely in vector control. Reproductive toxicity of resmethrin in the rat is also discussed.