Genetically modified mosquitoes carrying a repressible lethal trait have been developed as a possible means of control¬ling vector-borne diseases such as dengue fever, chikungunya, yellow fever and malaria. In Malaysia release of GM Aedes aegypti mosquitoes is being given serious consideration to control dengue, and a small scale open field release has recently taken place. The risks associated with this approach have been extensively discussed and documented, but there has been no similar attempt to dimension the associated benefits. This paper demonstrates the applicability of a semi-quantitative approach to risk-benefit analysis to compare risks and benefits of open field release GM mosquitoes taking into consideration environmental, health and socio-economic impacts.