RATIONALE: Evidence on short-term effects of ultrafine particles (UFP) on health is still inconsistent and few multi-center studies have been conducted so far especially in Europe. OBJECTIVES: Within the UFIREG project, we investigated the short-term effects of UFP and fine particulate matter <2.5 µm (PM2.5) on daily cause-specific hospital admissions in five Central and Eastern European cities using harmonized protocols for measurements and analyses. METHODS: Daily counts of cause-specific hospital admissions were obtained for Augsburg and Dresden (Germany) 2011-2012, Chernivtsi (Ukraine) 2013-March 2014, Ljubljana (Slovenia) and Prague (Czech Republic) 2012-2013 focusing on cardiovascular and respiratory diseases. Air pollution and meteorological data were measured at fixed monitoring sites in all cities. We analyzed city-specific associations using confounder-adjusted Poisson regression models and pooled the city-specific effect estimates using meta-analyses methods. MAIN RESULTS: A 2,750 particles/cm3 increase (average interquartile range (IQR) across all cities) in the 6-day average of UFP indicated a delayed and prolonged increase in the pooled relative risk of respiratory hospital admissions (3.4% [95%-confidence interval:-1.7%;8.8%]). We also found increases in the pooled relative risk of cardiovascular (exposure average of lag 2-5: 1.8% [0.1%;3.4%]) and respiratory (6-day average exposure: 7.5% [4.9%;10.2%]) admissions per 12.4 µg/m3 increase (average IQR) in PM2.5. CONCLUSIONS: Our findings indicated delayed and prolonged effects of UFP exposure on respiratory hospital admissions in Central and Eastern Europe. Cardiovascular and respiratory hospital admissions increased in association with an increase in PM2.5. Further multi-center studies are needed using harmonized UFP measurements to draw definite conclusions on health effects of UFP.