The aim of this paper was to analyse the effect of particulate matter PM(2.5), a recent air quality guideline value for the protection of health, on hospital admissions in Madrid, Spain. This dependent variable was used as a measure against the daily number of emergency hospital admissions from 2003-2005. The causes analysed were: all causes, respiratory and circulatory. The independent variables were daily records of PM(2.5), PM(10), NO(2), NO(x), SO(2) and O(3). Seasonalities, trend, flu epidemics, noise and pollen were used as control variables. Poisson Regression Models were performed to calculate the Relative Risk (RR) and the Attributable Risk (AR). The function relationship with hospital admissions was linear and without threshold. The RR for an increase of 25 microg/m(3) in PM(2.5) concentrations was 1.07 IC 95% (1.05 1.09) for all causes; for circulatory was 1.08 IC 95%: (1.03 1.13) and for respiratory was 1.07 IC 95% (1.02 1.11). PM(2.5) concentrations were the only primary pollutant that showed a statistical association with hospital admissions in Madrid.