Numerous studies have shown associations between ambient air pollution and daily mortality. OBJECTIVES: Our goal was to investigate the association of ambient air pollution and daily mortality in Erfurt, Germany, over a 10.5-year period after the German unification, when air quality improved. METHODS: We obtained daily mortality counts and data on mass concentrations of particulate matter (PM) < 10 mu m in aerodynamic diameter (PM10), gaseous pollutants, and meteorology in Erfurt between October 1991 and March 2002. We obtained ultrafine particle number concentrations (UFP) and mass concentrations of PM < 2.5 mu m in aerodynamic diameter (PM2.5) from September 1995 to March 2002. We analyzed the data using semiparametric Poisson regression models adjusting for trend, seasonality, influenza epidemics, day of the week, and meteorology. We evaluated cumulative associations between air pollution and mortality using polynomial distributed lag (PDL) models and multiday moving averages of air pollutants. We evaluated changes in the associations over time in time-varying coefficient models. RESULTS: Air pollution concentrations decreased over the study period. Cumulative exposure to UFP was associated with increased mortality. An interquartile range (IQR) increase in the 15-day cumulative mean UFP of 7,649 cm(-3) was associated with a relative risk (RR) of 1.060 [95% confidence interval (CI), 1.008-1.114] for PDL models and an RR/IQR of 1.055 (95% CI, 1.011-1.101) for moving averages. RRs decreased from the mid-1990s to the late 1990s. CONCLUSION: Results indicate an elevated mortality risk from short-term exposure to UFP. They further suggest that RRs for short-term associations of air pollution decreased as pollution control measures were implemented in Eastern Germany.