The purpose of life tables is to describe the mortality behav iour of particular groups. The construction of general life tables is based on death statis tics and census figures of resident populations under the hypothesis of closed demographic sys tem. Among other assumptions, this hypothesis implicitly assumes that entries (immigrants) a nd exits (emigrants) of the population are usually not significant (being almost of the same magnitu de for each age compensating each other). This paper theoretically extends the classical sol ution to open demographic systems and studies the impact of this hypothesis in constructing a life table. In particular, using the data of residential variations made available to the public by the S panish National Statistical Office (INE, Instituto Nacional de Estad ́ ıstica) to approximate migratory flows, we introduce in the p rocess of constructing a life table these flows and compare, before and after graduation, the crude mortality rates and the adjusted death probabilities obtained when mi gratory flows are, and are not, taken into account