Both causes and effects of trade union membership have been widely reviewed by economic literature. However, since it is difficult to find data,the determinants of trade union membership have not been analized for Colombian case. In his document we present the first estimation of the structural determinants of trade union membership for Colombia, which includes some specific features as region and economic sector effects, using the 2007 Integrated Household Survey (GEIH). We find that trade union density is determinated by similar factors to the ones reported by Johnson (2005) forother labour markets with similar collective labour bargaining systems.