The objective of this paper is to introduce a diVerent approach, called the ecological-longitudinal, to carrying out pooled analysis in time series ecological studies. Because it gives a larger number of data points and, hence, increases the statistical power of the analysis, this approach, unlike conventional ones, allows the complementationof aspects such as accommodation of randomeffect models, of lags, of interaction between pollutants and between pollutantsand meteorological variables, that arehardly implemented in conventional approaches. Design—The approach is illustrated by providing quantitative estimates of the short-termeVects of air pollution on mortality in three Spanish cities, Barcelona,Valencia and Vigo, for the period 1992–1994. Because the dependent variable was a count, a Poisson generalised linear model was first specified. Several modelling issues are worth mentioning. Firstly, because the relations between mortality and explanatory variables were nonlinear, cubic splines were used for covariate control, leading to a generalisedadditive model, GAM. Secondly, the effectsof the predictors on the response were allowed to occur with some lag. Thirdly, the residual autocorrelation, because of imperfect control, was controlled for by means of an autoregressive PoissonGAM. Finally, the longitudinal designdemanded the consideration of the existenceof individual heterogeneity, requiringthe consideration of mixed models. Main results—The estimates of the relativerisks obtained from the individual analyses varied across cities, particularly those associated with sulphur dioxide. Thehighest relative risks corresponded toblack smoke in Valencia. These estimateswere higher than those obtained from theecological-longitudinal analysis. Relativerisks estimated from this latter analysiswere practically identical across cities,1.00638 (95% confidence intervals 1.0002,1.0011) for a black smoke increase of 10μg/m3 and 1.00415 (95% CI 1.0001, 1.0007)for a increase of 10 μg/m3 of sulphur dioxide. Because the statistical power is higher than in the individual analysis moreinteractions were statistically significant,especially those among air pollutants and meteorological variables.Conclusions—Air pollutant levels wererelated to mortality in the three cities ofthe study, Barcelona, Valencia and Vigo.These results were consistent with similarstudies in other cities, with other multicentric studies and coherent with both,previous individual, for each city, andmulticentric studies for all three cities