Toxicological studies have provided evidence of the toxicity of ultrafine particles (UFP), but epidemiological evidence for health effects of ultrafines is limited. No quantitative summary currently exists of concentration-response functions for ultrafine particles that can be used in health impact assessment. The goal was to specify concentration-response functions for ultrafine particles in urban air including their uncertainty through an expert panel elicitation. Eleven European experts from the disciplines of epidemiology, toxicology, and clinical medicine selected using a systematic peer-nomination procedure participated. Using individual ratings supplemented with group discussion, probability distributions of effect estimates were obtained for all-cause mortality and cardiovascular and respiratory hospital admissions. Experts judged the small database of epidemiological studies supplemented with experimental studies sufficient to quantify effects of UFP on all-cause mortality and to a lesser extent hospital admissions. Substantial differences in the estimated UFP health effect and its uncertainty were found between experts. The lack of studies on long-term exposure to UFP was rated as the most important source of uncertainty. Effects on hospital admissions were considered more uncertain. This expert elicitation provides the first quantitative evaluation of estimates of concentration response functions between urban air ultrafine particles and all-cause mortality and hospital admissions.