In a broad sense the underground economy (UE) can be defined like productivity activities violating laws and regulations and defying political authority.The UE is a relevant problem in Colombia because the drug trade an informal economy activities are important issues in the Colombian economy.Not only this is true for the Banco de la República (Central Bank), because the impact of UE on cash demand, but also because its fiscal and institutional implications.The authors estimate the UE size for Colombia using the Kalman filter.Their approach starts from the assumption that underground transactions are paid in cash in order to make detection more unlikely.The size of the underground economy is reflected in the amount of cash used in a country beyond that used for official transactions.Due to the limited degrees of freedom, intervals of confidence through a bootstrapping procedure were calculated to establish the significance of the different UE "causes".